You can't do anything now BUT you can take them to any guest relations window at any of the parks after you arrive and make additions. If you want to upgrade to AP you can only do this at DTD or MK guest relations.
I was able to upgrade our tickets when we went in '03. We had a few days left and wanted to add on which they did without problem at WL. We had a PH and upgraded to a UMP. Maybe things ahve changed since then...