DivX is a video codec, NSV is just a container.
Supported video codecs for NSV container are On2 VP3-5, VP6.x, H.264
(H.264 decoding not supported by default - info here
Other than that, I doubt if you'll get better quality from converting.
You'd be best to start from the original format and encode to maybe XviD.
NSV (VP3-6/H.264) is predominantly used for streaming purposes, ie. half-decent quality at lower bitrates.
[Moved from Tech Support to NSV forum]