How hard would it be to convert a freshwater 20 gallon to a salt water tank??

Don't bother using the filters. You'd be better off getting some powerheads for movement, and then going with a DSB (at least 4 inches of sand) and then about 30 lbs of live rock. Much easier on you in terms of maintenance.

You'll need a hydrometer--vital bit of equipment for a SW tank.

Depending on what kinds of critters you want, upgrading your lighting might be required. Most corals require light for survival--they contain photosynthetic organisms in their cells. Without upgrading above NO lights, you can have a wide variety of shrimps and crabs, as well as a few small fish, hermit crabs, a small serpent star, snails. Sponges do well in low light tanks. Or, you could have just a bunch of live rock and find one of the smaller angler fish. Very cool critters--bright colors, not shy in dimly lit tanks, fairly hardy.