Like adding a consequence, I feel like adding stress boxes is the realm of powers and stunts, not spells. And there are reasons the stress track is as short as it is -- with this set up, I could just cast this spell a couple times, giving myself a dozen stress boxes, effectively unlimited spells, and then at the end go, "Oh, it takes me out. I wake up a couple hours later."
Also, once a character's taken out, they don't take more consequences. If you could take more consequences while taken out, why would you bother letting yourself be taken out?