Thanks for the help Benson and the clear explination!
I tried the cheater approach, but somenthing went wrong. I only saw in wireframe the nurbscurves, in open gl there was nothing. I will give it another try.
I dicovered the combine tool. Its great that it makes fluid transitions between two different nurbssurfaces. Really nice and easy.
Its a pitty that the push/pull tool and the normale extrude doesnt Worms on nurbes, do hou know why that is?