There is a playerpirate.entity file, and I'm pretty sure it has a line "canColonize FALSE" (the playables are set to true). However, given that pirates don't really have AI in the same sense as actual computer opponents, whether or not that's useful I don't know.
It's possible that you could stick some colony ships in their raid make-up, and if the canColonize flag works after they manage to destroy a planet, the colony ships might snag it. But again, the game may not be designed to handle pirate-controlled colonies, since the pirate base is a specific entity as well.
