Using your rig exactly as it is. Bones, bone names and orientations, weights, mesh names and blend shapes are exported unchanged. Add extra bones in the next step: only the area inside their influence radius changes.
Build any skeleton by clicking on the model. Each click adds a joint as a child of the selected one, so clicking along a tail makes a chain. Select an earlier joint to branch. Drag joints to adjust.
Joints with no children are end points: they shape the bone before them and carry no weights. Del removes the selected joint, Esc deselects.
For objects that aren't avatars: a dangling charm, an antenna, a cable, a tail on a hat. A Root bone holds the rigid part of the prop; chains attached to it are what PhysBones will move.
Root bone
The pivot the prop hangs from. Drag the yellow marker, or snap it.
Draw a chain