Would it be possible to take a 02 WRX turbo and put it in a 2.5 RS?

The turbo alone? Yeah.

The turbo and manifold and complete "kit"? Not without a WRX crossmember, water pump, and a few other things.

Would it be worth it, or would it be easier to just get an aftermarket kit?

It would be easier to get a kit, or what I'm doing is getting a turbo and making a custom kit.