As for the dragon you can convert poly to nurbs very easy and it usually looks better as a nurbs.
I've never seen a tool that can convert polygons to NURBS, and I've been doing 3D modeling for a long time. NURBS to polygons is easy enough, but polygons to NURBS is basically impossible.
Well, you can convert subdivision surfaces to NURBS, because SubDs are basically treating vertices as if they are NURBS control points, but that's not going to give you great results. And anyway why bother? The modeling is already done, the texturing will be totally different (NURBS are a lot less flexible about texture mapping,) and the end result looks exactly the same. Which leads into the other point...
I also don't see why you say the model would look better as NURBS. Not that polygons look better than NURBS either; my point is they look the same. It's just two different ways for the computer to store geometry data. Either way the geometry is drawn using the exact same renderer.