I think Tarquelne was joking
FF uses nifs, but you might have to make sure they work with the animations and so on.
but.. in principle, every model in a format you can import into blender or max,... can be used for CIV units, if you are willing to rig it to an exisiting rig or create custom animations.
Furthermore... FF and HOMM models do have a lot more polygons and bigger textures, so performance wise... not a good idea to use them.
On the other hand, i prefer doing original stuff, so dont ask me to do that.