So I can add the locations to that list, but is there a way to add the polygons? I see the ability to switch to a different mirror for the mapping polygons. If I did that, could I copy/fork the existing data and just add in the extra data I need?

The easiest would be to add it manually on the site:

I don't think we've created a process yet for updating the core files with grid and polygon data. I'll double check.