You can use the "attatch nurbs surface". But, it seems that the surfaces must be "clean" with not trim action performed on them.

Btw, you can untrim a surface from the nurbs menu!

If you pick the outermost isoparm on two nurbs surfaces, you can use the "attatch without move" to create an inbetween surface.

You have to do some trial and error on this one, as I have only recently begun to use this attatch surface tool.