I’ve got an older White Industries ENO hub which, if I remember correctly, is threaded for a fixed sprocket and locknut. Would it be possible to fit a freewheel to the hub?

yes, it just screws on to the sprocket thread.

Obv the locknut threading is superfluous, and sits at a narrower diameter inside the freewheel body.

I highly recommend white freewheels if you can afford them. But if youre testing ratios first, get a cheap indian/chinese freewheel to test.

You can get a BMX screw on freewheel

Thanks. I just thought the threaded part of the hub might not be wide enough to fully support the freewheel, but perhaps the stepped part which normally holds the locknut is narrow enough not to matter. I hope that makes sense!