This seems to be a perennial theme -- people wanting to turn good quality digi cameras into webcams.
I did try this with an old Fuji camera designed for that dual use and the results were no better than with a regular webcam (because the res has to be low for internet use).
Then there's the issue of power consumption -- most digi cameras need their own power when connected to USB.
Wouldn't it be more sensible to get a good webcam -- not that expensive after all.