You can add days as long as your ticket is still valid (before end of day when you use the second day) and isn't already the max length (5 days). Unless they've drastically changed policy recently, of course. But I've seen it discussed here somewhat recently.
Just go to the ticket booth to change your 2-day to a 3-day PH. Note that you won't get a MM, though. You have to buy the 3-day in advance to get that (can't be bought at the ticket booth).
You'll also potentially lose any savings you might have had on the tickets as you pay the difference between what you paid and the option you're upgrading to. You'll also be spending time in a ticket booth line to do so.
Note that only single day tickets matter what day. Multi day tickets do not use the same tier pricing that single day tickets do.