Well a custom map can be edited with a image editor like Photoshop but only the biome map and the heightmap - think of the map as a color coded filter to what will appear where and the smoothness of the mountain and plains range - In which case you would paint the biome map to your liking by adding the color white for snow, green for forest and yellow for desert. This will allow you to add these biomes on your map to your liking. The heightmap controls the amount of height in a map like valleys and mountains and adds the smoothing from these areas by contrast, where white will be up (mountain) and black will be down (at sea level) The contrast from smooth rolling hills to cliffs would be the color gray between the white and black.
Here is an example of the biome map:
https://i.imgur.com/SuYiyoN.png
The white, green and yellow define the colors of the snow, forest and desert respectively. So get out I MS Paint (which come with windows 10 I think) and I choose the color green with the color picker and paint some more green over the white, doing this added forest where snow was.
Example of the heightmap:
https://i.imgur.com/pVxaliq.jpg
So I do not like a cliff at one part where there is stark white to black, so I get out MS Paint and I smooth out the white and the black by making it more gray between the two, I just made a rolling hill where a cliff would have been.
There is a POI heightmap but I do not believe it to be a good thing to edit, not sure though, but you could ask on the forum about it.
These seem to be the only 2 editable parts of the Nitrogen map. The placement of POIs and such are viewable:
https://i.imgur.com/qNtMQZe.jpg
The traders can be spotted as reddish/pink dots and the towns, farms, cites and other buildings are obvious.
Here are the option you can pick and some have up to 6 choose-able options like: none, few, medium, default, many and lots.
https://i.imgur.com/Tzlt8ZV.png
It may be difficult to understand the "map" thing as being not really a road map but a visual reference of changes, I have been used to the term "map" in my days learning 3D graphics programs as well as using level editors for different games in the late 90s and early 2000s, where a map could be a texture map, or a bump map (which is really the heightmap in Nitrogen) or even a procedural map, which is controlled by adding numbers or other options (which would be the Nitrogen main interface) rather than an image. but not really the literal term of map as a roadmap in the real world.
Now this is really only from what I know about the tool and you could find out more on the forum post. I did adjust the heightmap once and added a lot of valleys, but that may not even be correct, because I only did it as a test and I did not really to keep the map to explore it completely.