DD went on a family trip last summer(6 days) and her senior trip (5 days) this year with a 10 day hopper and she needed an extra day. I tried to add a day to her ticket at DTD Guest Services during my January trip and was told that I couldn't add days to a 10 day ticket. DD tried again once she got to WDW this past May and was also unsuccessful. She ended up paying full price (ouch!) for that extra day. The policy might be different with a ticket that hasn't been already partially used though.