Having saved and assembly (or a part) as a part with only the outer surfaces, is it possible to fill it in so the part is now just a solid item? If so, how?? If possible, can I then cut into the new solid core?
If an assy was saved as a part using the Exterior Surfaces Only option, the surfaces would have to be made watertight & knitted together to be recognized as a solid. Then normal solid features may be applied.
You may also be able to use the Defeature function.