I've gone down that road, and I can assure you, it's not as easy as it sounds.
First you have to find the .im file you want to use as a kitbash piece, say a bell for example.
Then you have to copy said file, plus any associated textures, into the folder of the locomotive you want to use as your kitbashing platform, my example being a Union Pacific FEF.
After that, you have to go into the locomotive's config file and pick a spot where to put it. It isn't easy since if you get it wrong it'll tell you that it can't commit the changes because the file has the wrong kuid.
Anyway, it would look something like this:
mesh "examplebell.im"
auto-create 1
position 1.2,7.8,2.4
The left number determines sideways positioning, the center number determines the positioning along the body, and the right number determines vertical positioning.
It takes a lot of fine tuning to get it right, but the results are rewarding in the end.