Add a spell book ability to your item, this can add up 11 spells to one of the four slots.
While this is useful it only can do so much with certain items types, or items depending on use effect
Example spell book, opens a page on screen to show a list of spells
While having it on a claws of attack you can really only give passive effects.
Plus damage,armour,stats/auras/passive skills crit bash ect
Or do triggers once an unit acquires item add a list of functionality to do whatever you want, add spells, do a scripted event, change xyz ect.