Hello Sebastian,
>>>>>>>This doesn't work when importing a step assembly
Not in general - If the export supports the mentioned feature, the assembly structure will be reconstructed. That works also for sub-assemblies.

I have tested it actually with with SW, Geomagic and know it from my good old Catia V5 times (in deep mourning ...). I would test it also with NX(I hate it!!), but my actual employer uses an 3rd part converter:-(

Have a nice day
Karsten