I have heard you can on here only. There is info in the sticky on theme parks and attractions. I was leary of trusting that you can since I have also heard of some ppl having issue with it.
From the sticky-
http://www.disboards.com/showthread.php?t=1452684
I. Picking up Package Tickets Prior To Start Of Package
In cases where Walt Disney World Resort Guests would like to begin using their Package Tickets prior to the day of check-in (up to three days in advance), the Guests will be directed to visit any Theme Park or DTD Guest Relations location and request Early Ticket Pick-Up. (Note this cannot be done at a water park.)
It is only valid for people with packages at a Walt Disney World owned and operated Resort. You can be staying off-property at the time you pick up the tickets. Note that if you have a package which includes Dining, although you can pick up your tickets early you cannot get access to the Dining until you have actually checked in to your Resort.
Note that since August 2011 the Call Centers have been telling Guests this cannot be done. However it is still a valid procedure and can be done with no problem at Park Guest Relations Windows.
There is a possibility that if your package and the stay prior to the package are at the same Resort that you will be able to get the tickets at the Resort and not have to go to a Park Guest Relations Office.
And the tickets issued will be valid for the entire stay; they will not receive valid tickets when they actually check-in to their resort. There is a strong possibility that the tickets you have received prior to check-in cannot be added to your KTTW on or after check-in.
I would suggest you copy down the following information if you are planning to upgrade promotional tickets. If the person in the window is hesitant to upgrade these promotional tickets, ask them go go into The Hub >> WDW >> Work >> Ticketing >> Processes & Business Rules >> Early Ticket Pickup For Resort Packages where they will see the procedure.