Since both games use the same engine, I would expect that it would be as straight forward as making a new object set in the Geck and pointing said objects to the meshes you are importing.
same engine. but different versions. 20.0.0.5 and 20.2.0.7.
There are large differences. one major thing, everything now has a name in a string index list. So everything in the nif is, I guess, actually stored& thus read slightly differently. let alone all the completely new block types bethesda introduced to the nif format for F3, basically anything with BS. BSShaderPPLightingProperty, BSFadeNodes, some particle nodes, etc
Is not that simple. any oblivion nifs will need a conversion to a format consumable by F3.
Likewise, objects from FO3 should work with NV when it comes out, and Oblivion ones probably will too.
Highly speculative! and from past experience, they be always udating the engine, its unlikely. but entirely possible- we just won't know until we actually know