Not within Galaxy Forge. However, you can make your own random lists by making a mod and editing the GalaxyScenarioDef file. You can then paste this file into the GalaxyForge folder, replacing the one already there, to be able to use your new random planet lists in a map.

Without the mod that has the new planet lists , the game will not know how to handle a map that uses your new random planet types and will crash.

Also FYI, there is a random planet list already that will place either an asteroid or a moon/dwarf planet.