Yes you can add days by paying the difference between what you paid and the cost of the new "upgraded" pass.
example 3 day to 4 day
advance purchase 3 day hopper $139
gate price 4 - day hopper.........$189
Cost to upgrade......................$50
If you prepurchase the 4-day hopper you pay only $20 then the 3 day. So you are better off to just purchase the 4 day, if you think you might want another day.