I'm guess you're talking about WDW, as there aren't season passes at DLR, but I imagine as long as you can get into the parks on a day it's avaliable, you'll be able to purchase it. In my experience, you just go up to the counter and ask for the pin, then they'll ask to check your AP, and gladly ring you up. I don't see why they would put restrictions on pins depending on what pass you have.