Sure thing, any mouse can be adjusted for left-handed use, but whether lefties will find such device convenient - that is a different matter. Now many companies develop gaming mice for those who used to address challenges with their left hand from the very cradle.
Most mice for left-handers are ergonomic and can be found in vertical and horizontal versions. They have special finger seats. All main buttons are located on the right side instead of the left. The Razer DeathAdder Left Hand Edition can be considered a striking example of a mouse for left-handed players. Your cannot make your mouse truly convenient by simply readjusting it - better find one that will literally play into your hands.
LOGITECH was one of the first companies that decided to take care of the left-handers. That is exactly why they now hold leading market position. Many believe that left-handed mice are way more expensive, but this is a fallible judgment. Any gaming mouse will cost a bundle, despite the hand it was created for.
When choosing a device for lefties focus on standard gaming mouse features. Consider number of buttons, be sure to check their location. If you share your PC with another person also check the number of supported profiles. DPI is a critical parameter for any gamer as the higher it is the better your accuracy (shooters are best played at about 8,000 dpi).