I believe as long as it's before the last day of use then you can add whatever you want! (you can't add anything after you've used all your days - however I think you can add things during the last day up until midnight but don't quote me). I've added additional days (no WP&M but it was an addition to the ticket) the day before my last day and I was charged the difference between what the 7 day pass(what I changed it to) and the 5 day pass (what I had) costs.
Keep in mind they'll charge you the upgrade based on the initial pass, not how many days are remaining.