I can't view the mod assets you wish to convert. as mentioned no assets from other games can be used... if the meshes are not from or use parts of vanilla oblivion assets. then conversion is allowed.
you wouldn't have to start from scratch. basically- a simple import ob nif into blender/max. set up scene for export. export as f3 ver nif.
I can even coax animations between versions. mostly only useful for animated statics and stuff. anything with skeletons and full anim sets is a different kettle of fish.
You don't put a mesh into a nif, nifs are the meshes.
nifs are much more than simple mesh geometry... I can think of it like putting a mesh
into a nif. just semantics.