3d model addons "layering". Detail 3d models atop of existing 3d model

I’m curious if it will be possible to add 3d model “layers” to existing 3d models of planes. For example, I want to add elements like full passenegr cabin, or a coffee cup in the pilot cockpit, but I don’t want to remodel and reupload the whole existing plane. How do you do that and will it be possible? Can you add just a separate “layer” of a 3d model atop of the existing one? The same stuff Euro Truck Simulator had, where you could add minor detailing to the whole truck. Hope someone knows. Thank you!