Look for a 3D editor called Milkshape, and a set of plugins for NIF files by a dude called ChrisOfBorg (if memory serves). If you do use those plugins, prepare to have the smoothing groups messed up.
Alternatively, there's a program called NIFskope which can also help in the porting process, but sadly lacks comprehensive modelling tools. Perhaps some combination of the two. Milkshape is a nice starter editor with an easy-to-get-to-grips-with interface. It's ever-so-slightly buggy, so save your work often.