Been asking for this for a long time now, still hasn't happened
All my textures are going to waste :-(
But anyway, it may happen eventually, as they've stated they plan to eventually add the ability to add your own custom models and it's only logical that that would include textures, and if so, then it would also make sense to add the ability to apply your own textures to planets, but I feel they should do the planets thing first since importing models would probably be really hard to implement.