Of course. But you don't have to import anything into a bsa file. The files only have to be present in the correct folders (data/textures/ and data/meshes/ etc)
Especially for Fallout 3 -> New Vegas content you don't have to convert anything, so you just have to pick the texture and nif object and put it into the New Vegas folders, then open the GECK and create a new mod where you add the object, etc. etc.
Check out tutorials for adding new objects n' stuff with the GECK. There should be plenty around in the web.