Converted freshwater tanks are not the way to go, If you want a salt tank than buy a salt tank.... but to anwser your question, YES it is possible with the right equipment.
Small salt water tanks are difficult, as there is not much you can stock it with. Most fish will get to big for a 20 gallon. (remember the fish/gallon ratio for salt is about 1" of fish per 10 gallon) You could have a micro reef, but that would require some expensive equipment to make it successful, so i reccommend again getting a proper salt water setup from the start.
Basically its pointless to buy $400-$500 on equipment and put it on a $100 freshwater tank. I say this cause the tank itself is the most visible part of the aquarium, so why skimp on the main part, when your going to have to spend alot anyways.