Hi, @Jack_Zvaigzne,
Not every polysurface can be converted into a single surface, if that’s what you’re aiming for.
Especially if the polysurface is not continuously smooth and has trimmed faces.
I would recommend rebuilding the surfaces and simplifying the geometry to fit your further aims.
You could also try using meshes to shrink-wrap the polysurface (see here, and here).
A quick example attached below illustrates the issues.
polysurface grid_re.ghx (1.2 MB)
You might also find this tutorial useful to better understand how NURBS surfaces work.
