my problem is by far not that hard

I simple wanted to approximate a NURBS surface by a different bspline-surface. I wasn't aware FreeCAD has already a tool for that.
face.toBSpline(tolerance?, u_continuity, v_continuity, max_u_degree, max_v_degree, number_of_patches)
And this works quite good. But my method has one problem with this function which can be seen in the picture:
The problem is that this mesh has only boundary vertices and the result of nurbs.toBSpline(1, "C1", "C1", 3, 1, 10) somehow has degree higher then 1 and not only poles lying on the boundary in v-direction (direction from bottom to top of the cylinder). So the poles in the middle of the cylinder have no impact on the green mesh and are computed to zero.
To solve this I need a mesh which has vertices not only on the boundary, or a bspline-surface which has degree 1 in the direction of the straight lines. But it seems as this doesn't work with nurbs.toBSpline(1, "C1", "C1", 3, 1, 10)