How should I extend the wiring so I can move a 220v subpanel?
The junction box needs to be accessible, so "in the ground" cannot mean "buried without access."

In many cases it makes more sense to just replace the wire from end-to-end, but if you can put a junction box (that is accessible) either at the former sub-panel location or somewhere closer to the supply (so that you are shortening the supply wires before extending new wire to the new sub-panel location) that should also work.