It's been years since I last dabbled with RPG Maker, but looking at the code and the info comment, all you have to do is create a Steal skill and put <STEAL> in its note tag, and then you give that skill to your thief.
By the way that script is very cool. The steal effect is coded to trigger in addition to the other skill effect so you can put <STEAL> it on a physical attack and you can make a Capture command like Locke has in Final Fantasy VI. (One that rolls attack and steal into one action.) And looking at the code, it looks like you have a new steal chance bonus stat so you can create items that buff thieves. It also has other effect like pick pocket, or blue mage style ability stealing. I wish we had that back in RPG Maker 2000. (I had to hack a Steal command using RPG Maker event. )