CAD tip # 5117:

If you need to change e.g. 3DFACE (3D faces) to 3DSOLIDs, convert the 3DFaces first to surfaces with the CONVTOSURFACE command. If your source object is a polyface mesh (e.g. from 3ds max), first explode it to 3D faces.

Since version 2010 you can directly convert a "watertight" 3D mesh (polyface mesh) to a 3D solid. To keep the original unsmoothed facets, set the FACETERSMOOTHLEV variable to 0. Use the MESHSMOOTH command to convert the 3D mesh to a MESH object (if necessary) and then make the 3D solid from it using the CONVTOSOLID command (right-click on the mesh and choose "Convert mesh to").