Hi Andy,

re:
> So what I want to do now (for example) is create a NURBS surface over the top, maybe
> just part of it, and then extrude that. But how?

That's a process called "reverse engineering" and it's a pretty specialized and advanced type of use for CAD.

MoI is not designed to be a reverse engineering tool so it's not really a good fit to try doing this in MoI.

You might try Rhino, there are some reverse engineering tools made for it some info here:
https://wiki.mcneel.com/rhino/reverseengineering

- Michael