Don't expect too much for now, I don't think map modding is supported yet.

If it comforts you: I already counted the hexes and know the dimension of every map size, and am currently mapping out RL Earth with a grid viewer and a topographic map of the world...
... outside of Millennia, though... Unless map modding or a map editor becomes a thing in the future I'm wasting my time and no one will ever see it

Thank you Alfred, I hope they create the map editor.

I hope too ...

Is anyone still working on this?

The game uses special terrain cluster generative tools. Which is why you can write world types but not 1:1 copies.

1:1 copies in the current "endstate" of the game needs a little trick. You basically need to number all tiles of a map border (north or south), in rising order. Then based on adjacent IDs number the other. Through this you give the entire map grid unique IDs and could then apply a pre-coded map onto those IDs.

The generation process would likely take 5 minutes, but it is doable. Writing this numbering algorithm hasn't been done though and thus adding a custom map with guaranteed outlays is not possible right now.

It's doable though.