Grasshopper nurbs surface manipulation

i have a planar surface constructed from an interpolated curve boundary. I would like to form a gentle depression in its centre - ie just want it to sag in the middle keeping its boundary planar
Instead of converting to a mesh surface and manipulate it via Kangaroo, is there another way to do this in Grasshopper?option A2 main (4.5 KB)

Hi Cshea,
you need to internalize your input points for others to see your file working.
It sounds like something that can be done just by moving control points?

I attach the rhino file with the points, the gh file and image
does that help?

option A2 main surface.3dm (372.9 KB)
option A2 main (4.5 KB)