Hi Anthony - some other NURBS modelers have a tool for this situation called "Patch" or "N-sided patch".

Unfortunately MoI does not currently have this tool, but I do plan on incorporating it in a future version.

The way the N-sided Patch normally works is that it calculates a larger rectangular surface that fits through points that are sampled from those curves, then that larger rectangular patch is trimmed to that outline.

NURBS surfaces themselves have a 4-sided layout to them naturally, so you can't actually have a single "natural" (meaning untrimmed) NURBS surface with 6 sides like you've got there. But you can have a 6-sided trimming boundary that cuts away material from a larger surface.

It is possible to construct this in MoI as one single surface if you manually do the same approach that the Patch tool would do for you - build a larger surface and them trim it back. But it is hard to get a symmetrical result in this particular case. Here is a description though in case it helps you with any ideas.

First I used Draw curve / Freeform / Control points to extend curves so that they met in corners:



Now you can do a sweep that creates a larger surface:



And then trim back:



Some method related to this (create a bigger surface and then trim back) is necessary if you want to make a single surface that has an irregular boundary around it.

The other way is to try and build this with multiple surfaces. That will definitely be a way to fill it in with surfaces, but right now MoI does not have controls for maintaining smoothness between adjacent surfaces though, so it will be hard to create a super smooth result with that method.

Hope this helps,

- Michael