On a track hub the narrow width of thread that takes the fixed sprocket is slightly too narrow to accomodate a single-speed freewheel. A multi-sprocket freewheel would be far too wide, though. I remember when I was just getting into cycling in the early 60s I did run a 3-speed freewheel for a while on a double-sided fixed hub, but I felt uneasy about it, and obtained a second hand wheel that took a freewheel as soon as I could.