As far as I can see, it should be possible to disconnect the Vin (5volt) wire from arduino and replace the unused 9-volt battery, with a 12 volt dc power supply.
Thew l293 uses 2 positive voltages, one of 5 volt max on pin 16 (of the chip) to power its internal logic and pin 8 is used for the power needed to run your motor. You'll need to disconnect the wire to Vin, to avoid blowing up your arduino board.