There are other sharpeners on the market, for serrated edges. Basically, they're all similar in concept, either very narrow/round or triangular in order to fit into the serrations on the blade.
Other creative means could be employed, like wrapping sandpaper around a dowel or small screwdriver, etc.
Edited to add:
I've seen a post here, from a good while back, from somebody who used a Dremel with a felt buffing wheel & green compound to 'power strop' the serrations on a blade. As long as the serrations don't need any major repair, that sounds like it'd work pretty well too.