The custom levels is built using the custom editor (the regular ones are built using Unity's own editor). If you wish to have an editor where it is possible to load external models with textures, please write one and let me know, I'd be interested.
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.