Should be possible but it all depends on the rear spacing of the fixed gear frame. Many track and fixed gear bikes are spaced very narrow at 120 mm if they are true fixed gear frames, you may be dealing with a converted cycle. The Shimano nexus hub requires the rear spacing to be 130 mm so be sure to check the frame before purchasing any parts. As far as the dropouts are concerned any horizontal should work as they just need to be there so that you can tension the chain since there is no derailleur.