How do I chop a not hat?

I have been making characters from a game I like in the sims. Some of them have very specific cc, and fortunately someone has made that cc, but it only works without issue with the other parts of the set. Ie, a specific hair for a hat. One of them has a robotic arm, and it is in the glove category and disappears when a shirt with sleeves is added. I've already figured out how to properly move it to tattoos. But I'm stuck on figuring out how to make it work under clothes with sleeves. I tried to mirror the settings on a detail for the arm that's in tattoos, but it only ended up setting one of the textures to mirror the shirt sleeve texture. I did research and I'm pretty sure I have to split the mesh so it is compatible with the different sleeve lengths. But I don't know how to, since it would have to be backwards to how you do it with pants, because it is the accessory that is being split rather than the pants. 

I've asked permission to edit the cc already, but I don't want to post the files or encourage editing of it without permission. The closest thing I could find for use in testing would be the arm from discover university but separated from the shirt and as a glove.