I am not an expert on Universal, but we are going in June so I have begun to do some research. My understanding is that if you get either the Preferred or Premier AP, you get 10% off additional ticket purchases. If you get the multi-day passes, I believe you have to use them within 14 days of the first use-- I don' t think there is a no expiration add-on option like Disney.
Hope this helps, and if someone knows more, I am sure they will come along soon!!
Next Up: 10/12 BC
BLT, CR; OKWX2; BCX4; YCX3; Poly; PORX2; WLX4; PopX3; DolphinX3; ASX3
HGVC I-Drive, Seaworld & Parc Soleil, Hilton Bonnet Creek, Gaylord Palms, Omni Championsgate, Royal Pacific, Portofino Bay