I showed up to the ticket booth with my 5-day passes, explained that I wanted to take my kids for an extra day. George (the ticket guy) said he couldn't guarantee anything, but if I went to guest services and asked, it might be possible.
It was possible! I am one happy Canadian mom! Sure enough, Mary (the guest services lady) upgraded our passes to 6-day park hoppers. The difference in price was $135 for one adult and two kids, much less than it would have been to go their normal route and purchase additional 1-day park hoppers. Considering that park hoppers recently went on sale for $30 off each, I don't feel too bad.
SO, all you have to do is ask!