Here is a useful routine that will create isometric blocks based on an existing 2D (flat) block. This is great because all that you have to do is create the one block and this routine will do the rest for you.

Here’s how:

• ISOBLOCK <enter>
• Select the block that you would like to create an isometric view of
• Toggle through the isometric views by either hitting the spacebar or the <enter> button
• Once you have the correct Isometric view, press any letter followed by the <enter> button

To create another isometric view, insert the original 2D block and repeat the above steps. Also note while you are in the insert block dialog box, that as you make new isometric blocks, they are given unique names.

Link to original post at AutoCAD Tips

Pictured below: Existing 2D blocks

;;-----------------------------------------------------------------------
;;
;;  Command Name - IsoBlock
;;      Routine For Transforming a Block to Isometric
;;      By WizMan_07Feb10
;;
;;  Version 1.0 - 11May09
;;  Version 1.1 - 06Feb10 - Added Reverse Option and Flatten(Express)
;;  Version 1.2 - 07Feb10 - Fixed DText Rotation inside block(by SEANT)
;;
;;
;;-----------------------------------------------------------------------
;;
;;
(defun c:isoblock (/          blok_ent
                   counter    ent_data
                   ent_pt     i
                   sub_func   *error*
                   blk_name   midtbox
                   midtxt     reverseflag
                   rot        tbox
                  )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun to_southwest (ent_name base_pt / obj)
        (vla-TransformBy
            (setq obj (vlax-ename->vla-object ent_name))
            (vlax-tmatrix
                (list
                    (list (/ (sqrt 2.) 2.) (- (/ (sqrt 2.) 2.)) 0. 0.)
                    (list (/ (sqrt (/ 2. 3.)) 2.)
                          (/ (sqrt (/ 2. 3.)) 2.)
                          (sqrt (/ 2. 3.))
                          0.
                    )
                    (list (- (/ (sqrt 3.) 3.))
                          (- (/ (sqrt 3.) 3.))
                          (/ (sqrt 3.) 3.)
                          0.
                    )
                    (list 0. 0. 0. 1.)
                )
            )
        )
        (vla-move obj
                  (vlax-3d-point
                      (trans (cdr (assoc 10 (entget ent_name))) ent_name 0)
                  )
                  (vlax-3d-point base_pt)
        )
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun to_southeast (ent_name base_pt / obj)
        (vla-TransformBy
            (setq obj (vlax-ename->vla-object ent_name))
            (vlax-tmatrix
                (list
                    (list (/ (sqrt 2.) 2.) (/ (sqrt 2.) 2.) 0. 0.)
                    (list (- (/ (sqrt (/ 2. 3.)) 2.))
                          (/ (sqrt (/ 2. 3.)) 2.)
                          (sqrt (/ 2. 3.))
                          0.
                    )
                    (list (/ (sqrt 3.) 3.)
                          (- (/ (sqrt 3.) 3.))
                          (/ (sqrt 3.) 3.)
                          0.
                    )
                    (list 0. 0. 0. 1.)
                )
            )
        )
        (vla-move obj
                  (vlax-3d-point
                      (trans (cdr (assoc 10 (entget ent_name))) ent_name 0)
                  )
                  (vlax-3d-point base_pt)
        )
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun to_front (ent_name base_pt / obj)
        (vla-TransformBy
            (setq obj (vlax-ename->vla-object ent_name))
            (vlax-tmatrix
                (list
                    (list 1. 0. 0. 0.)
                    (list 0. 0. 1. 0.)
                    (list 0. 1. 0. 0.)  ;mirrored
                    (list 0. 0. 0. 1.)
                )
            )
        )
        (vla-move obj
                  (vlax-3d-point
                      (trans (cdr (assoc 10 (entget ent_name))) ent_name 0)
                  )
                  (vlax-3d-point base_pt)
        )
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun to_front_southwest (ent_name base_pt / obj)
        (to_front ent_name base_pt)
        (to_southwest ent_name base_pt)
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun to_front_southeast (ent_name base_pt / obj)
        (to_front ent_name base_pt)
        (to_southeast ent_name base_pt)
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun dtr (var)
        (* PI (/ var 180.0))
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun fix_txt (blk oblang / ins)
        (vlax-for
                  obj
                     (vla-item (vla-get-Blocks doc) (cdr (assoc 2 (entget blk))))

            (if (eq "AcDbText" (vla-get-Objectname obj))
                (progn
                    (Setq ins (vlax-get obj 'insertionpoint))
                    (vla-put-upsidedown obj 0)
                    (vla-put-ObliqueAngle obj (dtr oblang))
                    (vlax-put obj 'insertionpoint ins)
                    (vla-update (vlax-ename->vla-object (entlast)))
                )
            )
        )
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (defun *error* (msg)
        (if blok_ent
            (progn
                (load "flattensup.lsp")
                (acet-flatn (ssadd blok_ent (ssadd)) nil)
                (cond ((= sub_func (quote to_front_southwest))
                       (fix_txt (entlast) 30)
                      )
                      ((= sub_func (quote to_front_southeast))
                       (fix_txt (entlast) 330)
                      )
                      (t nil)
                )
                (if reverseflag
                    (vlax-for
                              obj
                                 (vla-item (vla-get-Blocks doc) blk_name)

                        (if (eq "AcDbText" (vla-get-Objectname obj))
                            (vla-rotate obj (vlax-3d-point midtxt) pi)
                        )
                    )
                )
                (setq reverseflag nil)
            )
        )
        (and doc (vla-endundomark doc))
        (setvar 'cmdecho 1)
    )
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (setq doc (vla-get-activedocument
                  (vlax-get-acad-object)
              )
    )
    (vla-EndUndoMark doc)
    (vla-StartUndoMark doc)
    (setvar 'cmdecho 0)
    ;;
    ;;--------------------------------------------------------------------
    ;;
    (if (setq blok_ent (car (entsel "\n>>>...Pick a block...>>>: ")))
        (progn
            (setq ent_data (entget blok_ent))
            (setq ent_pt (cdr (assoc 10 ent_data)))
            (setq blk_name (cdr (assoc 2 ent_data)))
            (to_southwest blok_ent ent_pt)

            (setq counter 1)
            (while (or (= (setq i  (strcase
                                       (getstring
                                           "\rPress [SpaceBar] to Toggle View, [R]everse or Press any Letter to exit: "
                                       )
                                   )
                          )
                          ""
                       )
                       (= i "R")

                   )
                (if (/= i "R")
                    (progn
                        (if blok_ent
                            (vla-delete (vlax-ename->vla-object blok_ent))
                        )
                        (setq sub_func
                                 (nth counter
                                      '(to_southwest
                                        to_southeast
                                        to_front_southwest
                                        to_front_southeast
                                       )
                                 )
                        )
                        (entmake ent_data)
                        (setq blok_ent (entlast))

                        (if reverseflag
                            (vlax-for
                                      obj
                                         (vla-item (vla-get-Blocks doc)
                                                   (cdr (assoc 2 (entget blok_ent)))
                                         )

                                (if (eq "AcDbText" (vla-get-Objectname obj))
                                    (progn
                                        (vla-rotate obj (vlax-3d-point midtxt) pi)
                                        (vla-update (vlax-ename->vla-object blok_ent))
                                    )
                                )
                            )
                        )


                        ((eval sub_func) blok_ent ent_pt)
                        (if (< counter 3)
                            (setq counter (1+ counter))
                            (setq counter 0)
                        )
                        (setq reverseflag nil)
                    )
                    (if (not reverseflag)
                    (progn
                        (setq reverseflag t)
                        (setq rot (vla-get-rotation (vlax-ename->vla-object blok_ent)))
                        (vla-put-rotation
                            (vlax-ename->vla-object blok_ent)
                            (+ rot pi)
                        )
                        (vlax-for
                                  obj
                                  (vla-item (vla-get-Blocks doc) (cdr (assoc 2 (entget blok_ent))))

                            (if (eq "AcDbText" (vla-get-Objectname obj))
                                (progn
                                    (setq ins (vlax-get obj 'insertionpoint))
                                    (setq tbox (textbox (entget (vlax-vla-object->ename obj))))
                                    (setq midtbox (mapcar '/
                                                          (mapcar '+ (car tbox) (cadr tbox))
                                                          '(2. 2. 2.)
                                                  )
                                    )
                                    (setq midtxt (mapcar '+ ins midtbox))
                                    (vla-rotate obj (vlax-3d-point midtxt) pi)
                                    (vla-update (vlax-ename->vla-object blok_ent))
                                )
                            )
                        )
                    )
                        )
                )
            )

        )
    )
    (*error* "")
    (princ)
)
(vl-load-com)
;;
;;--------------------------------------------------------------------
;;
(prompt
    "\n>>>...IsoBlock.lsp is now loaded. Type 'IsoBlock' to start ...<<<"
) ;_ prompt
(princ)
;;--------------------------------------------------------------------
;;
;;WIZ_07FEB10