No but galaxy forge can do that when the creation of planet by making that particular planet owner the militia, can be selectable by the toolbox.
Pirate raid? or Pirate base?
Pirate raid composition gameplay.constants has to do with this one.
Pirate base are modified via GalaxyScenarioDef.galaxyScenarioDef which they both located in gameinfo folder.
Militia are located same in the file of GalaxyScenarioDef.galaxyScenarioDef.
Make sure the items number are match with item you add or your game crash.