You cannot add militia with just galaxy forge because there is no "RandomMilitia" owner option. You have to save your completed map in galaxy forge, then open the .galaxy file in notepad, find your groups of militia and manually change the Owner section to "RandomMilitia".
Just because I know it works, you may want to look at a already made custom map like mine that uses a lot of custom militias to see how the file looks in notepad.