If it's a freehub, uh, hub (pretty sure it is) then you can use the surly fixxer. But because of the vertical dropouts, you'll still have to play around with cogs and rings to find the "magic" gear to keep the chain tight.

If you want more available gear ratios, you'd have to use a hub with an eccentric axle like the White brothers ENO hub. At $135 for just the hub, you're losing a lot of the appeal of a fixed conversion vs buying a dedicated single speed frame with horizontal dropouts/track ends.