I don't think you can. And it is for good reason too. If you want to be a warrior type, it shouldn't be easy for you to cast heal spells.
I don't like this philosophy at all - it seems to just be classes by another name (ie, the game telling you what a certain type of character absolutely can or can't do, at the expense of integrity and gameplay).
That said, I don't think the actual mechanic is designed to force warriors to never be healers (or archers never to use swords, and so on), but rather for the sake of the control scheme and general balance of item/ability use. A good, fluid system for swapping out item/spell sets will completely remedy the problem in any case.