To do it in-engine, you’d probably have to go through the mesh one part at a time and paint the bone weights so that each rigid body part is only using one bone. This can be time consuming, but it’s really not hard, per se.
To speed things up, personally I would suggest exporting the bendy mesh UE generated, and editing it in Blender (or some other 3D software) so you can select vertices by surface and quickly assign them to a vertex group instead of manually trying to paint things which might be overlapping in the ref pose.