Yes, its really a single speed. Just, not a very good one. With the chainline not perfect, you run the risk of tossing the chain. But it won't hurt too bad, as long as you keep tension good. The best solution is to take off the 5 speed freewheel, respace/dish your hub/wheel, and then thread on a bmx freewheel. That is a totally acceptable way of doing it. Here's how (look for item #3):
http://www.fixedgeargallery.com/wheels/

The procedure is exactly the same whether you're going fixed, or freewheel.

Have fun, and be safe!