manually set power for pop up flash on 40D?

I am currently using a Nikon D60 in the studio and I manually set the popup flash to 1/32 power and use it to set off my strobes (camera in manual mode as well). I am considering switching to Canon equipment, either a 40D or 50D for now and want to know if I can (in manual mode) vary the power of the popup flash on either of these cameras. I downloaded the manuals for both and don't see any reference to manually setting the power of the popup flash. Does anyone know if this is possible? TIA.