I don’t think you can but not 100% sure. Last May I had to do it myself daily after park entry. But this years tickets are different. So maybe? You’d have to phone and hopefully get someone who who knows.I'm torn! We are going to book our trip soon, likely as a package through Westjet Vacations. They are offering Canadian Resident Tickets at what seems to be a flat CAD rate on the current deal that ends up being a few percent cheaper than direct through Disney. Only issue is that they don't offer adding MaxPass to all tickets ahead of time. Not sure if they would be able to add it at the ticket booth when we would go to validate our Cdn resident tickets/exchange the vouchers.