>> That's a lot more difficult because at least with Sub-d there are a lot of areas that have the exact conversion.

>> With these other types, there is no exact conversion anywhere, so the entire thing has to go through a kind of fitting / optimization type process.

There's all sorts of clever ways to go from unstructured stuff like voxels to NURBS without necessarily getting into super complex fitting algorithms. Here's one "cheap and dirty" =) way I can think of doing it.
This method would run into problems when a model has a hole or cavity that goes from one side all the way through to the other, or with something like a tree where things branch off from the main trunk and need to be handled separately. But even that could be fixed with a bit of clever coding I think and the user himself could be tasked with segmenting or color coding the voxel volume so that it can be parsed easily by the NURBS conversion algorithm.

I think this is one potential method for creating a bridge between truly 3D freeform sculpting that feels like shaping things with a soft modeling foam or gel, and more structured NURBs modeling. It would be enough if you could rough out a 3D organic shape quickly with a voxel brush/nozzle and get an "OK" quality NURBs model or set of curves out of it that can serve as a guide for the construction of a cleaner, better defined NURBS model.