Is it possible to sharpen a serrated knife?

The general answer is yes. The only trick is that the sharpening method depends on the kind of serrations. You can find a lot of videos on YouTube that explain how to do it. The easiest and cheapest method is to use sandpaper on a mousepad. You can start with 400 grit sandpaper and finish with 1000 grit.

In the case of knives serrated on one side only, you apply the sharpening procedure on the non-serrated side.

First, you put the blade on the sandpaper with a 20-degree inclination, approximately, which is the typical edge slope of a table knife. Then you slide the knife sideways, in the opposite direction of the blade edge (if you slide it in the direction of the edge, you may damage the sandpaper). 10 times with 400 grit and 10 times with 1000 grit should be enough for having a sharp edge, but it may vary depending on the steel hardness and the pressure you put during sanding.