I only have two freewheel hubs in my collection, but it was the same situation with them, only about 1 thread left after a cog went on. If my hubs were threaded all the way to the end, there might be enough for a BB lockring, but they're stepped down to an unthreaded section for the outer 1mm or so. Not sure if that's common or not.

If you're concerned, buy or build a wheel with a proper track hub. It's among the easiest wheels to build.