I was able to do it in May but it was a big hassle and took well over an hour to do so. We decided on our last day we wanted to add one more because there was a snafu in the computer on our first day in the parks that caused our 4 day park hopper to get charged for 2 days. So on our last day in the park (an early entry day) we got in line to be one of the first in for EE and they wouldn't let us in because it said our hoppers had been used up. Took about 20-30 minutes I think before we got in and by then all the FL rides had lines or weren't open (AIW ride was closed because the CM said "she was late"). At lunch DW and I talked it over and decided we didn't want our last day to be a bad one so we went to city hall on Main street to see about adding a day and complain about missing out on our EE. They tried and couldn't add on in CH so they took us out to customer service window outside the park and had them try it. It took a while but they finally figured out a way to do it without it costing 50 bucks per ticket (I think it ended up being like 10 bucks a ticket or something like that). They made it sound like that wasn't something they do anymore which is why it was so hard to get the computer to do it.
So Im not sure if they will or not now. I sure don't want to try it again after the last time though.