What about adding militia planets? I tried messing around with the planet templates, but none of militia ships or structures I tried adding were there...
I think Harpo has a quicker way, but here is how I do it.
1. Edit the planet you want the militia to appear on and add the desired amount of forces/structures.
2. Remember that planet's name, and save your map. Remember where you saved it, find the .galaxy file, right click and open with notepad.
3. Find the planet you wanted to have the militia around. You should see a list of all your added units. Change the owner to "RandomMilitia" (in practice you can also use "Pirate")
4. If you want the militia to own the planet (needed for structures to appear), set the colonize chance to 1.0
5. Save the file and the independent colony should be their.