Yes, it would have to be a guest relations agent at the park entrance. The ticket windows and hotel guest services agents are like airline check-in agents. The park guest relations agents are like airline GATE agents -- they have ALL the power, whereas the others can only perform certain functions.
The problem with adding days (and PH, probably) is that you really never could "add days". What they'd do is sell you a new ticket and credit you for the old. Since they don't sell non-expiring tickets, there isn't a price for them to either credit the 8-days or sell you the 10. I could see them being able to add PH, but not adding days.