Hey Andreas,
thanks for asking that question. I am afraid there is no obvious way to convert a hybrid subdiv nurbs model to a subdiv. Subdivision surfaces is a technique that has benefits in many areas, but it doesn't support trims, fillets, or other details, that can be realized with general nurbs surfaces. That is, why Alias provides you with a hybrid modeling approach. To get you the best out of both worlds.
Now depending on your requirements, there might be solutions though:
1. In case you just need the smooth tessellated mesh, you can export it in the mesh format you need, e.g. obj format. However, that will remove the possibility to modify the mesh, using the coarse control cage. You will only retain the tiny tessellated triangles. This can be useful for downstream processing like rendering, 3d printing or cnc milling. However, it won't be useful for modeling anymore.
2. In case you need the coarse control cage to be able to modify it, but this time it should include all the details like trims and fillets, there is no way around retopologizing the model. There are automated algorithms, that do retopologizing for you. Maya provides such an automated algorithm. Retopologizing a detailed hybrid model comes along with quality loss, though. So don't expect this to be exact, especially, when there are tiny details combined with large surfaces like in a hybrid model. Automated retopologizing is often sufficient in media and entertainment, when a perfect surface flow isn't business critical. However, when it comes to class A surfaces, a semiautomated retopologization is often better. Alias provides you with the Retopo Tool, which is made for that semi automated task. It will speed up the process dramatically. However, depending on the model's complexity it is still a fair amount of work. The gained quality is typically better, though - depending on the skills of the designer, of course.
Hope that helps
Stephan
