I've heard it might be a bad idea. Something about the engine not being strong enough to handle a turbo or something of that nature. The best recommendation is to drop a 3SGTE from an AllTrac in there, from what Ive heard.
If you goto
www.gtfour.com, theres an article explaining how to turbo a non turbo celica, and it says to swap the engine for a 3SGTE.