Can you put a 03-06 WRXSTi motor in an 02 WRX?

Yes but it requires certain modifications to do so. You need to swap the throttle body and all of the sensors from the 02 WRX to the STi block so that the ECU gets all of the proper signals. Ideally, you would want to consider using the WRX heads on the STi block since the WRX heads flow better than the STi heads.