Split Surface, manipulate edge, re-join

Hey everyone, I'm just starting to learn Grasshopper. I'm Trying to write a script that will allow me to manipulate a landscape that opens and closes, rises and falls to create space above and below. So far I have a script that Will allow me to manipulate form by lofting lines and removing sections by splitting.

My next operation I want to split the surface and manipulate the edge of the surface to create entry points to the spaces below the surface. I just can not figure out how to do that.

If I was modeling in Rhino I would split it the surface, extract Iso and edge curves, adjust edge curve, then use networksrf to reconstruct the surface. 

Any help would be awesome! I've included a screen shot of what I'm trying to do and my current script.