Move points on surface instead of project them

Hi all,

I'm using Grasshopper to create a model from a 3D scan of a body part. For that i project points on this scan, on z = -1 or z = +1 directions, depending of the position of the point from 3D scan, I create interpolated curves from these points and I loft these curves.

I need some points not to be projected, I need them fixed in space, but I can't know which ones by advance, it depends on 3D scans will be used, different each time.

The problem I find is that if a point is not projected it isn't created, so my curves network is corrupted and unusable.

Is there a way to use original point instead of projection in cases where points are fixed ?

Another way would be to move points on surface instead of project them, but I don't know how to do that...

Hope I was explicit enough.

Thank you by advance for your help.

Jérémie