I have been give 3D autocad file in the past and SW has been able to convert them to a part file but they come in as surfaces. Also where it is an assembly SW makes some arbitary 'guess' at which surfaces belong in which part/sub-assembly.

Is there a way to change this behaviour and is there a way to get the enclosed boundaries to automatically form a solid?

(These are dwg/dxf files with 3D information, not sat, iges etc).