How do you combine external models to create aircraft variations?

I am trying to use a base model with the bulk of the aircraft 3d model and then attach in game other files with just a couple of extra pieces to modify the base model and create multiple variations.

These file would be including livery decal variations, floats, skis and similar type of things…but mainly for livery variations that require geometry.

I haven’t found any clear documentation or tutorial on how to do this…

Can you tell a model to use several external files combined?..

Thanks…