How can you make an ability re-add itself after using it (casting at will)?

I'm playing through IWDEE for the first time, and I really like how some things are implemented as opposed to BG (and those beautiful visuals! seriously, get rid of BG's animations for fireball and cone of cold, use the ones in IWDEE!).

I loved how the paladin can detect evil *at will*. That's how it's supposed to be, unlike BG's system of just accumulating a lot of uses (which is capped at 50). I was wondering if any one of you could explain if you can easily do this for any spell using Nearinfinity, or is scripting required? I have many ideas about custom spells and importing these kinds of functionalities to BG, but I don't know what I need to know.