Not sure if you're still working on this or not, but I found some info for you by accident.

Go to virtual MPF. Click on Self Service Actions. Click on Separations. Click on Request to Change/Withdraw voluntary separation.

There it lists the reg that governs this shiz and gives you the electronic form to fill out for your request. It says it needs to be routed through your wing cc and says something about a letter stating your justification.

Good luck.