Hi,
I've tryed numerous attemps to do so (3d faces to 3D volumes), but I guess (from my point of view) it's impossible :

• export from SU to dwg/dxf and try to convert this 3D geometry in autocad to a solid one (using "convert" in autocad)
• export in 3ds, tried the same in Blender, 3DSMax, etc.
  I've not tried generic exporters/converters, but if you find a solution, I'm interested in (in a SU to ACAD worflow for civil plans)
  Cheers