You can use part of a teams harness as a single - it really depends on the harness you have. Mostly all you need is a single pair of reins as they are often the only part that is different, but again it depends on the harness you have.
It really all depends on the type of harness and what kind of cart you have. I am sure that it will work but you may have to improvise a way to attach it to the cart. Most single harnesses I have seen attach to the cart with hoops on the side to run the poles into (sorry, don't know what they are called).
However, most team harnesses are attached using tugs that run from the collar to a double tree and neck yoke snapped to the front of the collar then attached to the tongue of the wagon.
It may take a bit of jerry-rigging to get it to work but it is expensive to buy a whole new set of harness. :)