It's unlikely I'll add support for importable local systems (in the short term), but there's a template here that shows how to create new systems that I can integrate with a pull request on gitlab. If you're not a coder you can also email your ideas to isocondev@gmail.com!
Thanks for answering! I'm primarily an RPG dev myself (You might have heard of WARDEN), but I do dabble in coding and stuff, though not with Unity (for a looooong while). I'm just really rusty with C# so I thought I'd ask if there was a simpler method in the works.
Another thing: I seem to be only able to expand the map area on X-axis (i.e A-X). Is there some hotkey to flip the expansion tool?
There's not, but it's a good idea. Map expansion works by cloning a row/column, so currently the best option is to clone the first row/or column and then edit it down. Obviously that's a bit of a headache if you've got a lot of editing already done on that first row and need to terraform. I've also got a todo to make the map rotation controls work against the visual center (right now it pivots around the center of the base 8x8 map, which is pure oversight on my part).