I had the idea of trying to make a new race that's home planet was a few low PQ asteroids. After some trial and error it dosnt seem possible to change the planet to an asteroid like the ones in game. At least not a way I can think of. But I have been able to come up with an outcome that is coming along the right track. I thought it might be a good idea to post my progress and things I have discovered in case it would be helpful to anyone. Or if anyone had advice for some of the problems I still face.
So going though the steps that didnt work if people are interested:
-I first tried to just make a new planet in PlanetDefs changing the <ArtDefine> to AsteroidArtDef_01 which directly referenced the current asteroids. Then making a custom system in StarSystemDefs that referenced that new 'planet'. However this did nothing and the planet would default to an randomly generated base planet.
-I then tried to change the <BodyDef> to NormalAsteroid as per how normal asteroids are generated in a system and it just made the planet invisible.
-I then adding to PlanetArtDefs the relevant material information copied from AsteroidArtDefs which just gave me back errors when I tried to start the game. This I assume is due to the schema files, since the game dosnt know what to do with the extra data I have added.
Steps that ended up working with some trial and error:
-Finally I tried replacing in PlanetArtDefs under <ModelFile> to the location of the asteroid model. This did in fact change the model in game to a single asteroid, tho not a cluster seeing as that seemed to be generated in game and is not actually a single file. I felt like I could work with that if I could just change the texture as I would be happy with just one large asteroid, which I could easily change its size with <Radius>.
-I then tried both removing and replacing the various texture <AtmosphereTextureFile> , <BaseLayerTextureFile> etc in PlanetArtDefs with the textures of the asteroid. Although I wasn't able to use the same texture that the asteroids use as I think it is generated in game, I was able to get something close by making a custom gradient map while using the mercury base texture. I plan to make a full custom texture for it later.
-I was able to remove the clouds by making a new texture that had a completely black alpha map, effectively making them invisible. If I just removed the entry the game would automatically add a randomly generated cloud system.
-Then was the issue of the continent map, if I removed it a randomly generated one would appear. So I ended up directly copying the mercury map and just blacking out random areas in its alpha channel. Then I saved it as a new texture and it worked near perfectly except the colors didn't match, which i worked out was simply due to <BaseLayerDiffuseColor> under alpha. It was defaulted to 150 which I just set to 255 meaning the planet texture wouldn't have an altered colour so now matched seamlessly in game.
-For some flavor I experimented with the idea of changing the moon model to a small asteroid as well. You can see that in the below pics. I still have to make its texture the correct colour and resize it. But in principle it should work with no issue.
Other problems that are impossible (unless others have ideas) to overcome atm until the modding system is updated on stardocks end:
-The mapping on the asteroid model, which you can see in the below screens, looks terribly stretched and warped.
This is due to the fact that the models for all asteroid model files don't have a correct UVUnwrap. As there textures are normally pre-generated in game this normally isn't an issue, however it is a major factor when wanting to mod their textures. This would be an easy fix if we could either edit or add our own 3d files but currently there is no format we can add that GalCiv3 can understand and read.
-I also have not been able to add the "dust particle clouds" that is on all asteroids. I cant seem to find a way to make it render at the location of the planet but I havnt exhausted all my ideas on doing that yet. Although I suspect it wont be possible currently.
-As others have mentioned, its not possible without crashing the game or throwing out errors to append information into systemdef and a few other xml files with a mod unless you directly override these files in the main directory. Without being able to bundle them into a mod that wont override the main files it will mean ill have to hold off and doing much more with it atm. I mean, unless people still want to use it anyway once i get it looking good and working correctly. However i feel that ideally this just needs to be addressed by stardock instead.
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So thats my progress so far, im sure I have missed some points but there was so much to write.
Below you can see some screens of the progress. As mentioned in my post im still going to be tweaking sizes, colours, ect. But it gives you a general idea of the direction. Hope something I wrote was helpful to someone 
Pic.1 You can see the bad seams/uvs on the asteroid to the left. You can also see the continent map i was able to edit to make it seamless in the planet view.
For more information about making an invisible continent see my post here: https://forums.galciv3.com/472238/page/1/
Pic.2 You can see the planet model changed to an asteroid and the color/texture changed to match the near by asteroids.
Pic.3 This was a quick attempt at seeing how it would look to change the moon to an asteroid. Besides the colors and size it worked well i think.
