The Battles of Destiny map editor actually shipped with the game, but was not accessible via the DOS Box. GOG has fixed this, and you can now make your own maps! Instruction for using the Map Editor start on p. 61 of the manual, and I plan to livestream some examples of how to use it.

There is an image attached showing the new startup screen. If you do not get this option, uninstall Battles of Destiny (you will be able to keep your saved games) and reinstall it.