Hi. Yes, it is possible to edit the Middle-earth map.
Just find the map.png in one of the subfolders of the assets folder, inside the mod .jar file. You can get into the assets folder by right-clicking on the .jar (The one you download and put into your mods folder.)
Once you find the map.png file, drag it onto your desktop and edit it in a paint/photoshop software. Then redrag it back into the .jar and replace the old map.