Thanks for bringing that topic. I create new designs basing on 3d scans often. I use Zbrush, Modo, Rhino, MoI3d.

I have some thoughts about workflow. Maybe it doesn`t help you directly but I hope it will be helpful.

I think that not always is worth to move scans from polygon to nurbs. If you want thicken and trim it only it`s super easy in most polygons apps. I would do this in Modo or Zbrush but it`s also easy on Blender or MeshMixer.

I had one work few years ago where I decimated scan mesh in Zbrush and imported to MoI3d with Max Smirnov OBJ importer. Next I`ve created curves with using "on surface" snapping to create solid model after. This was no super exact but good enough to make that specific hard surface model. This model was prepared for CNC machine and they demanded STEP file. That`s why I have not made it in polygons.

May I ask why you need that surface in NURBS?

Sometimes I do retopo by hand over scan in polygons to have subd cage and I import it to MoI3d (native subd importer or scrip[t). If surface after import is not good enough then you may build new one over that surface in MoI (second retopo in nubrs this time).

Sometimes I sculpt scans in Zbrush before doing auto retopo (remesh) to have better surface for remeshers.

Sometimes I cleanup mesh errors before I bring it to other software (self intersections, ngons, non-manifold edges and polygons).

Sometimes I use remesher to leave all bad topology behind. Sometimes it is so bad that only manual method is possible.

I think CAD is not friendly enough for meshes to play with it so I try prepare meshes earlier in more friendly environment. But it`s my approach. Maybe it`s bad.

Sometimes I do quad remesh (there is free instant meshes but I prefer Zbrush Zremesher). You can import that quad cage as subd cage into MoI3d and it will also convert it to patches.

 

When I see so organic shape as yours I have so many questions in my head. Why nurbs? Is it prepared for CNC or for 3d printing or maybe only for render?