Well, SubD is new and not even officially available inside Rhino.The included converter still is in extremely raw state. It essentially replaces every single face in the SubD-cage with a Nurbs surface. More mature converters such as Tsplines try to create a usable patch-layout but no product outputs good Nurbs models in the sense that one may perform demanding surfacing operations on the converted model. If the goal is to cut some holes and possibly to create some simple fillets here and there this will do though.
This is the Zbrush sample dog converted to Nurbs with the V6 converter and with a precise hole cut into its belly. You can try this out right now by loading a quad mesh to the Rhino 6 Beta. Run the SubDFromMesh command, followed by the ToNurbs command.
If that all sounds too raw for you may use Autodesk Fusion for Nurbs conversion but that is a subscription program.There’s quite a bunch of other Mechanical CAD and dedicated Reverse Engineering programs which let you convert quad-meshes to Nurbs but they are very expensive + a lot of of them are subscription based these days. None creates a Nurbs model which allows for advanced modifications.
I can’t speak for McNeel but I dare saying No.
There’s no program which lets you freely convert models back and forth between SubD-meshes and Nurbs.
Rhino 6 won’t let you do this either.
Good news is that you can sculpt any Rhino model inside Zbrush. With Rhino 5, today.
I think you are mixing up techniques and terminology here. A SubD cage is a mesh with a particular topology with just four sided faces. Not all models with just quadrangles are SubD-Caged though. Meshes with such topology are ideal for smooth subdivision. But Zbrush will work just fine with any sort of less regular meshes as well. If the input is really bad there’s great ways for remeshing. If you want to bring the modified model back to Rhino you may simply re-import (I usually have no problem with the full resolution data and rarely have to use decimation). If you really need a Nurbs conversion you can create a conform mesh inside Zbrush.