What exactly are you trying to import?
I usually delete the masters and rename Fallout3.esm to FalloutNV.esm. But then, again it's just with some simple non-gameworld changing mods. :sadvaultboy:
I tried this and the mod displayed the UNKNOWN master file in the GECK when i tried to load it. I just want to import buildings, or the layout of them. no scripts, lights, or anything else. just machinery, tables, scaffolding, protectron pods. Its just way to intricate for me to care to redo it all.