It can be done at Guest Services at any of the parks. It only takes a few minutes. When we went in November I had 2 vouchers and needed 2 additional Passes and took care of everything when we went to the parks for the first time.
I think you can activate them at the hotel and maybe Downtown Disney but I wouldn't do it unless you plan on going to a park the same day. You never know you might need that extra day the next year.