Long time ago I asked MM if they could add asteroid colonies, because I saw that the info was put into one of the files... but info I mean max pop, fac., ect. The reply I got was, Not gonna do it cause there are enough planets to colonize.
The on question I have is why can't you put asteroids around a planet?? I tried with the map editor, but it makes the planet a moon. It was kinda funny, a huge GG as a moon around a tiny asteroid field.