Adding days to tickets with package

[mmouse50]

I currently have a package through Universal and have been wavering on the number of days and/or adding park to park access to my current ticket selection. Can I change the tickets when I get to the resort? I will be staying at the Hard Rock and wondering if I could change my tickets on my arrival for more days in the parks or just add park to park to my existing tickets with the package and pay the difference then.