I still think you can do it from a Ding fare by clicking on the Apply Travel Funds button once you go to reserve - it says you an use up to four unused Ticketless Travel reservations so I take it you can just pop in your existing ressies # (see below). I believe my friend did this by calling SouthWest and they did it for her. I'd call SouthWest though before trying.
From the SouthWest site:
Apply Travel Funds
You may apply funds to this reservation from up to four unused Ticketless Travel reservations, up to eight Southwest LUV Vouchers, and/or up to four southwestgiftcards. Funds must be entered one at a time. Electronic Funds are applied toward purchase in the order they are entered.