Sadly this is a bit of a pain to do, but
1. Open the group editor on desired planet.
2. Disable the use default template checkbox.
3. Create a new group with all the ships and/or structures (if you want the militia to own this planet) you wish.
4. Set the if Condition to either always or no owner (I don't remember which) and the Spawn for "NoOwner"
5. Now, save the map and go to the folder where it is saved. Right click on the map you made and open it with Notepad.
6. Scroll/search through the file to find the group you made, and change the NoOwner parameter to RandomMilitia.
7. If you added structures, set the colonize chance for that group to 1 as well.
Hopefully I didn't forget anything, but that should do it.