How To Get DWG Solids Into Inventor?

Does anyone know how to import 3D solids from a DWG or DXF file into Inventor as solid bodies? I know that you can import 3D solid entities from DWG/DXF files into Inventor, but they come in as surface bodies without volume. Which means that you can make topological modifications like fillets, chamfers and move faces. But solid modeling operations that require closed volumes like extrude, shell, hole, etc. are not possible on surface bodies.

I did find a workaround though. You can stitch the individual faces of the surface body using a Stitch Surface feature, after which solid modeling operations work. I am not sure whether this is the best way to work on DWG solids in Inventor, but this method works. I wonder why Autodesk does not simply import DWG solids as solids in Inventor. Both products use the same underlying modeling kernel, right?

I even created a simply box in AutoCAD. When I got it into Inventor it was a surface body. Maybe I am missing something here. Anyone?