If your tickets were NOT purchased as part of a package, just take them to Guest Relations and they can upgrade you to the Annual Pass with DVC discount.
If your tickets are part of a package, I'm not sure. The resorts don't sell Annual Passes, but Guest Relations can't upgrade tickets that are part of a package - the resorts do that.
My advice, if you have a package, is to just ask at the front desk.
Or, if you can, book room only, cancel your package, and purchase your annual passes seperately.