I don't really have a whole lot of confidence that the reverse direction of starting with NURBS and going to Sub-d is very feasible in a robust dependable way.
It could be possible for some special cases like when you have untrimmed surfaces. But once you have done any trimming or boolean operations, that will put trim curves on the surfaces, which means that the control point structure of the underlying surface can no longer just be used directly to convert over to a sub-d mesh, instead a completely new set of quads have to be created that attempt to follow the trim curves of the NURBS surface.
Then even after that, Sub-d surfaces are quite sensitive to the topology of the control mesh - if there is any irregularity to the topology it will result in things like ripples and bumps in the sub-d surface.
There is a good overview here on the different kinds of artifacts that are caused by poor sub-d topology:
http://guerrillacg.org/home/3d-polygon-modeling/subdivision-topology-artifacts
It's very difficult to make a clean topology with a totally automatic method, particularly with CAD models that may have a mix of sharp edges and smooth pieces as well as a mixture of small and larger features within the same model.
Here's a simple example - here I have created an extruded NURBS surface and then trimmed some areas of it away.
How would you then expect for that surface with the holes in it to be converted into a sub-d mesh?
It's going to be very difficult for an automated tiling mechanism to produce a very nice topology on that, especially in areas where 2 trim curves come close to each other.
And as you can see in the video linked to above, defects in sub-d topology result in things like ripples and bumps in the sub-d surface itself, so a conversion that generates poor topology will tend to mutate the shape and potentially introduce defects in what was previously a totally smooth extruded NURBS surface.
If that kind of shape mutation doesn't bother you, then your project probably fits more naturally just being done with sub-d from the beginning rather than in NURBS.
- Michael