Here's one approach.
Let' say you have two AutoCAD drawings. One contains the 2D views and the other a 3D model.
In the 3D model go to a viewing angle that shows the solid model in the isometric orientation you want.
Set the visualstyles to 2D Wireframe
Set DISPSILH to 1
Give the HIDE command and select the geometry.
Use CTRL-C to copy the lines to the Windows clipboard.
Go to the 2D drawing and give the PASTESPEC (Paste Special) command. This will paste an image of the selected geometry into the other drawing. SInce it is an image and not AutoCAD entities the image quality will be limited.
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If you want to paste the image as AutoCAD entities I have found you can use paste-special to paste the geometry into Word using the Picture (Windows Metafile) format. You can then ungroup the vectors, edit, group and paste-special back into AutoCAD as lines. Let me know if you want more detail on this process.
~Lee
lee.minardi