Why would a 1999 BMW M3 that was in a rollover accident have white smoke coming out of the exhaust when the car starts and after it is warmed up?

Not having anymore info. other than it was in a rollover, the engine head could be cracked and allowing coolant to get into the combustion chamber which could explain smoking at start up and after the engine is warm. Check your coolant tank for any oil residue - if there is oil in the tank, you've got a engine head problem and your best bet is to get a new head after verifing that the block isn't also damaged.