Well, there is not built-in NURBS functionality in Xith3D. But it shouldn't be too difficult to add that, if you know/understand the formulas, that NURBS are defined on. I suggest to search Wikipedia for that (here
is a german version. Didn't find the english one right now).
In principle you only have to compute the function value for each vertex of a grid shape.
I guess, it would be best to write a GLSL program and apply it to an org.xith3d.scenegraph.primitives.Grid.
Does this help?