For converting mesh to nurb, it is a process we called Reverse engineering. For organic shape,there are 3 softwares you can use.
1.Rhino with Tspline 2. Fusion 360 3. Geomagic studio (Using Tspline and Fusion 360 are almost the same)
My workflow is using zbrush to fixing the scanning data and using zremesher to control the number of polygon and topology.
Then export the obj. file to rhino. Convert it to tspline object, toggle smooth mode, convert tspline to rhino object. ( just like this tutorial : https://www.youtube.com/watch?v=WnU17armaXE)
But this method has several limitations , it may not accurate and may have slightly different between mesh and nurb . Second, it seems like there are limitation of the number of polygon . This method only worked while the polygon number below 100,000 as my experience.
Using Geomagic studio is the best method as i know and widely be used in the industry. U can easily check the deviation between mesh and nurb surface and no limitation of the number of polys. The most important thing is u can adjust the surface patch or export to other CAD software (Solidworks , UG NX…)
(Reference video of geomagic : https://www.youtube.com/watch?v=5B5PHKtTBEY)
But if your model is simply , i recommend that using rhino with Tspline (fast and simple)