You can manually change each of the asteroids into moons.
I have been playing with the idea that instead of replacing the asteroids as moons change the mesh of one planet to include a moon, or do it the cheap way, create a map for yourself using the distant stars mod to create a planet with a nearby moon (it has a moon planet, and a moon planet type) remove the asteroids, strip the logistical slots, strip the health research, strip or keep the planet pop research, and most of all strip the defensive and the starbase capabilities for the moon planets and then manually using the map creator planets the moons and create a map.