Is it OK to concrete some path edgjngs in when it's below freezing?
Not ideally, no.
Casting concrete in sub-zero temperatures (unless you can pre-heat it & keep it warm with thermal blankets) is a no-no.
Water expands when it freezes, & this "blows" the concrete.
The path-edgings (pre-cast I assume) will be fine, but not the concrete you set them with.
If you must do it immediately, because, say, you have a nagging wife, cover with blankets or some form of insulation. You can protect the concrete the same way, too.