I have the same issue. I have already existing chcaracter in my game with already existing skeleton, meshes and animations. I wanted to make a modular character out of it. I downloaded the same model with skeleton and separated the body parts+joints in 4 different bodyparts. I exported each of those body parts, but with the whole skeleton.
When I’m trying to import it and chose the skeleton (already existing in project) I see the same error that unreal can’t merge those parts with the skeleton I selected (the skeleton the body part has is completely the same as I’m trying to asing it to).