You can freeze egg yolks. If you’re likely to use them within three days of cracking open the shell, you can safely keep them, covered, in the refrigerator. For longer storage, you can keep them in the freezer for up to four months.
Sharon Tyler Herbst, author of several cooking reference books, including The Food Lover’s Tiptionary (silly title, useful book) says that yolks become gelatinous when frozen, so gelatinous that they can be hard to use. She recommends that you mix 1/8 teaspoon of salt or 1-1/2 teaspoons of sugar or corn syrup with every four yolks you freeze to inhibit their becoming gelatinous. If you try this, make sure you label the container clearly, so that four months from now you know whether you are supposed to be making custard or Hollandaise.