You can fit a fixed sprocket onto a freewheel thread. but you can't use a regular track lockring. You can however use a bottom bracket lockring as they use the same thread. As its not an opposing thread it's not as efficient but if tightened enough it does work, after all, almost every other type of locking nuts go onto the same thread. The only thing is there is a limit to how small a cog you can you because the BB lockring is bigger than the track one. I know a 13T, from personal experience, won't work this way, not sure what the smallest size that does work is.
You can fit a fixed sprocket onto a freewheel thread. but you can't use a regular track lockring. You can however use a bottom bracket lockring as they use the same thread. As its not an opposing thread it's not as efficient but if tightened enough it does work, after all, almost every other type of locking nuts go onto the same thread. The only thing is there is a limit to how small a cog you can you because the BB lockring is bigger than the track one. I know a 13T, from personal experience, won't work this way, not sure what the smallest size that does work is.