I am not sure to understand since I don't know much about B-Rep format structure, but I would say that your importer should stay as close as possible to the original file structure.
From what I understand, Brep format only contains a list of faces ( = trimmed nurbs surfaces ).
Does it also contain objects like shells or solids ?
Is your question more about the additional curves that lay across the faces ?
I don't know what is the real nature of these curves. It looks like they are only isocurves for visualisation purpose.
Anyway, I don't think we can go very far with nurbs surface editing until FC gets OCC's Geom2d implemented.
Right now, only untrimmed nurbs surfaces can be edited. We don't have full access to the boundary edges ( 2d curves in the UV space of the surface ).