I don't understand the exact science of it (have not cared to, LOL) but you can have a skimmer. It will not be effective in the same way it is in salt water however, so no real advantage. It's something about the fact that it is salt water and not fresh water that makes it effective in a marine application.
freshwater isn't viscous enough for the micro-bubbles to form, so the surface area created is quite small, which limits the overall effectiveness of the skimming.
this is not to say that skimming doesn't work on freshwater. I cant remember who posted details (with pics) about their setup. whereas saltwater skimmate is generally a dry foam (if setup correctly) with freshwater it ends up as a dark liquid.
have to ask why, personally I use surface skimmers to remove the protein film, which its very effective at doing. water changes are more effective in FW tanks for removing DOC
(remember that a lot of the filtration methods in SW is designed to reduce the overall DOC because salt is expensive)