u don't need TTL for external flash. one way to get the right combination of aperture/shutter speed is to meter the scene using aperture priority mode with the flash turned off. then set to manual mode with the aperture/shutter speed and activate the external flash.
Then have your fren stand in position and start taking pic, before the shutter close, just manually flash your external flash at them. if too harsh, add stofen omnibounce, or tilt the flash and use bounce card to bounce. hehe..
Opps. just realised what i said is exactly the same as mpenza hahaha