??? You mean a cassette style cog? Freewheels and cassette/freehubs are completely different. If you mean to make a singlespeed fixed-gear, while the cog will thread on to the freewheel side, there won't be the reverse threads for a lockring that'll keep it secured.

You can buy a singlespeed freewheel that'll thread on as a replacement to your existing freewheel. White Industries makes swank singlespeed freewheels (~$90-100), though there are much cheaper alternatives made by ACS, Odyssey, and Shimano (~$20).