There are some aftermarket manually operated safeties out there for Glocks. But no, I would not buy a Glock, or any other gun for that matter that was to be one of my carry guns, with a manual safety on it. This is one reason I like DAO designs. I want the least amount of switches, levers, and buttons on the gun as possible since all I want to have to think about in the event I ever have to use it is to pull-and-pull (pull the gun and pull the trigger).
I my opinon, Kahr has the best DAO out there if one wants a DAO and a safe gun to boot. Glocks are hybrid DAO designs (yes, both Glock and the ATF class the Glock design as a DAO) and are excellent for their purpose. However, I would not carry a baby Glock in a jacket pocket as I would, and have, a Kahr pistol.