You need to set a master pose component.
This can be done in C++ or Blueprint.
In GB, our character’s mesh is used as the head and master component.
Legs, torso & hands are attached at runtime and then have SetMasterPoseComponent() called.
The one thing to remember is to create a physics asset for your master component that covers the whole body.
i.e. Our physics asset has physics bodies for spine, legs, arms etc, despite being just a head.