Okay I recently bought a fixed gear from a guy off of my local craigslist. After buying I a realized that the rear cog is screwed directly to the rear hub with no lockring.

So my question is can I simply add a lockring to my original rear hub? or do I have to buy a new hub or wheel. The tool at my LBS told me I had to buy a new wheel, but then again he was a total toolbag.