The problem with organic geometry conversions always is the huge number of resulting faces that 3D CAD software really can't handle. But there is a (largely unknown) solution for that: T-Splines.
As I understand it, T-Splines is not intended for poly to nurbs conversion. The poly model is used as a control "cage" to modify or deform the nurbs model in ways you can't do with nurbs tools.
While T-Splines does produce a nurbs model from polys, it can only do so with a very low res, or decimated version. I believe 500,000 polys is the max it can handle.
If you need to convert millions of polys to nurbs, you will need something like Geomagic.