Yes it is possible! Not common, but it CAN happen.
The white coloring happens because of a recessive gene that can be hidden for several generations before making an appearance. Both parents must carry the white gene to produce white German Shepherd puppies.
It is very possible that two "normal colored" German Shepherds (carrying the white gene) can produce white puppies in a litter of black and tans or sables, etc.
A white German Shepherd is not albino, and should also have both dark eyes with a black nose and lips. White is also considered a "masking" gene which covers up the dog's true color, although it does not affect skin pigment. This is why "white" German Shepherds often have a cream or blond tint to their solid white coat.