I have quite a bit of nurbs experience in maya as I modeled exclusively in nurbs for a long time.

Unfortunately your only tool options are the round and fillet blend tools under the edit nurbs options.

These tools are notoriously flaky!

If you look at the built-in help for the tools you can see some examples of how to use them. They work for me about 30% of the time though.

Also with trimmed nurbs surfaces (which is what you get when to boolen nurbs objects) surface render tesselation is critical or you will get all kinds of gaps between your surfaces.

but to be honest I am not a big fan of maya's nurbs surfaces toolset. It is a grossly watered down version of what was in the professional nurbs surfaces package from alias called studiotools and does not have to robustness to handle anything but the simplest of filleting tasks.

Looking at the surfaces you are trying to blend between I can see you they are going to be almost impossible to blend using the round and filleting tools.