That is something weird as your levels wasn't maked in editor, you limit possibilities of your product... just add possible to load external models with textures and coordinates to spawn.
I would keep it simple... community can make levels in Unity's own editor... then all problems are gone... I think that it will be a lot easier to make levels in Unity as I can do modelling in whatever software I want and load it to Unity's editor and then save them as Unity's and simply load them in FPV Freerider... The editor is there for us all... and it's free... no need to complicate.