I think I know this answer, but if I buy a 6 day ticket (1 park per day) can I add on park hopper for the last day? I know I can not do that when initially buying the tickets, but will they let me add that once I am there?
You will end up paying the park hopper charge for all the days of your ticket if you decide to add it. You cannot add park hopper to one day of a multi-day ticket.