Here are two posts that talk about adding additional days
http://www.disboards.com/showthread.php?t=2773898
http://www.disboards.com/showthread.php?t=2803629
Hope that helps. Just a word of warning, my brother-in-law tried to upgrade from a 3 day to a 4 day Parkhopper and they were able to see how much a paid for the 3 day (less then the current rate) and the upgrade was the cost of a current 4 day pass minus the cost of what he paid for the 3 day not what the current cost of the 3 day was. Hope that makes sense. So hopefully since you got a good deal on a 6 day pass they will just look at it and see that it is a 6 day pass and charge you only $5 per person to upgrade to a 7th day.
We went from 5 to 6 days for $5 per person. We decided halfway through that we didn't want to leave DL area for the day at the end of our trip like we had planned. We just asked our hotel a few days into our trip and they changed our tickets for us. I know that's different than going to 7 days
