Dxf import and cap holes

If you want to work like a CAD guy in NVil (rather than an imprecise artist). You can use the geometry of the mesh you imported to snap NVil splines to edges and vertices. Its pretty quick to do.

You can then use spline surface or spline patch to patch the areas between the splines with the amount of divisions you require. Spline surface requires 4 splines to patch while spline patch will join onto existing mesh edges but still require 4 sides. (Not much different than nurbs requirements where they require 4 edges to surface).