Newbie Equipment Question

I have a 6 speed older cassette with a double 52/42 from my old bike. I recently got a Cannondale CAAD8 frame and everything is fine, except my braze on shifters. The new frame doesn't have downtube bosses. Can I substitute a brake lever/shifter (9 speed) for now, until I change my drive train? Will it work? I can't afford to buy a whole new wheel set and drive train all at once.