Yes, quite a few mods are available for Diplomacy, just look through the mod forums and check the mod threads and you should find some.
If you want to add stars/planets from mods to the galaxy forge, the easiest way is to just copy the galaxysenariodef file from the gameinfo folder of the mod and paste it into the galaxyforge folder (coping over the one that is already there). The new items won't have icons, but they will appear in game when the mod loads the map.
