The real answer is "maybe".
Can a serrated knife be sharpened? Well, since they make serrated knives, and they need to make them sharp before selling, obviously yes.
Can serrated knives be sharpened by their owners? That will depend upon the knife, the serrations, and the equipment available to the owner.
Some serrations can be matched adequately well with readily available ceramic sharpening rods. Some are so narrow or deep that it is hard to impossible to find sharpeners to suite.
Chris