Guest relations and the tickets booths cannot help with this. The only way to use a WDW ticket is to just take it to the entry turnstiles, where they will push a button to call over another CM who will take your ticket and remove a day from it, and then bring it back to you. This will take a few minutes, and you'll just wait over to the side. When they return, they'll give you a ticket to use as your ticket for the day in fastpass machines.