Serrated knives can be sharpened, but unlike ordinary knives, they need a diamond-coated steel that's properly sized to match the knife.
If you want to master the technique, take your serrated knives to a knife or cookware store and ask to have your knife matched to the correct diameter diamond-coated steel. Its diameter has to match the scallop profiles. If you have more than one serrated knife, choose a tapered steel that'll work for different scallop sizes.
If you examine the blade, you'll see that one side is tapered and the other is flat (this is called a chiseled edge). The tapered side is the only one that gets sharpened. When you're through sharpening, drag the knife through a scrap of corrugated cardboard to knock off any leftover filings.