Hi kittonian,
a looooong time ago there used to be an add-on for AC 9-10 called MaxonForm that would do exactly what you're asking for.
You could import several file formats, edit them with pretty much all of C4D modelling tools available at the time, and then convert them to editable (in terms of materials) AC objects. You could also create new objects from scratch.
If you ask me, it was the best addition to AC of all times.
Unfortunately it has never been updated for subsequent versions of AC, even though Nemetschek has acquired Graphisoft. A real pity.
I still have a legacy mac running 10.6.8 (last version with Rosetta) and AC 9 just to be able to use MaxonForm.
I know that doesn't help you that much, but every now and then I try to bring MF back in the discussions hoping that someone would be brave enough to revive it
😉
Cheers
Francois
François Chatelain
Worldwide Digital Imaging
Formerly posting as RanXerox
"A little bump will help blur your reflections"