You cant share the full model I think but theoretically you could share only the parts to modify the airplane (via merging the two files in MCX) like in the second picture but I dont know if RATS or FAIB would be allowing this either.

I made my own mode with the use of blender. I took the normal RATS model to blender and modeled the rest of the plane.

A mod designed in Blender to turn the RATS IL-76 model into a IL-82.