You're going to need Blender's Nif Scripts found here : http://niftools.sourceforge.net/wiki/Blender -Make sure you have the exact versions of each of the requirements also. (Blender, Python, PyFFi, Nif Scripts) After those you should be able, in Blender (With your model), to go through File -> Export -> NetImmerse/Gamebryo( .nif & .kf & .egm ), for a successful .nif format export.