I don't think you can, I think you can flatten the assembly structure as per the attached image by editing the import settings, but then you would need to export a parasolid of the assembly and then import the para solid back in to a clean part file, this will leave you with a series of solids, but no assembly.
Cheers
Si
Best regards
Simon NX 7.5.4.4 MP8 and NX 8.5 (native) - TC 8 www.jcb.com