Just get a cog that has the same number of teeth as your freewheel and thread it on. A track hub has a smaller set of reverse threads to hold a lockring. The lock ring keeps your cog from spinning off when you apply back-pressure.
If you just want to try fixed, just pull off your freewheel and thread on a cog. Just keep on your brakes.
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I'm not one for fawning over bicycles, but I do believe that our bikes communicate with us, and what this bike is saying is, "You're an idiot." BikeSnobNYC