No that will not be possible unfortunately, for technical reasons all models/textures etc. needs to be built into the editor itself. My goal is to make it as flexible as possible though.
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.