Sure if it's drilled with overflows for saltwater you can use them and use a sump or wet/dry for filtration or plug them using bulkheads and use other kinds of filtration.
If it comes with equipment though, things like a skimmer you won't need for freshwater.
Also if it was a saltwater tank it will have to be scrubbed down before setting it up for freshwater to make sure no residue is left. If it was a reef it will really have to be scrubbed down as there are inverts that attach themselves to the side etc and leave a heavy residue.
I agree w/ cichlaguapote, although most FW D.A.S are done as heavily planted tank's w/ little to almost no fish (http://www.aquariumadventure.com/tipsheet04.htm). These type tanks are gen. pretty expensive because of the preinstalled filtration that they offer. In your case, you decide if you're going w/ the sump or not & rather you will use any other part of the built in filtration C02? ( kinda hard to see it all w/ the pic.'s) & then just cap the remaining blkhd's that you wont be using. When cleaning just be sure to stay away from the silicon when scrubbing, let it soak for a bit B4 you clean it w/ a bit of aquarium salt, helps losen up & break dwn the depsits. Med. to firm plastic scrub brush.
Are you getting a great deal or something on the tank to be considering it? Wouldn't be my 1st FW tank choice unless the price was, well almost free.