Michael,

There is no direct way to import DWG files into SH3D, not as a basis for further drawing, nor as a background image.
You can try to import a DWG file into Blender, then export it as an OBJ file, which in turn can be imported into SH3D. Note, however, that such an OBJ file cannot be edited in SH3D, because it will function as if it were a piece of furniture.

Hans