The nut is anything to hold the strings off the neck at the head end and keep them from sliding sideways. Traditionally it's a rectangular rod of bone with some notches sawed in it.
A brass button-head slotted screw is traditional nut for a banjo fifth string.
Here you see a "zero fret" and some nails acting as the nut.
I tried to play my accidentally left-handed banjo but it hurt my brain. So I reversed the strings back to right-handed order. Then I reamed a peghole on the correct side and hung the fifth string there, with a nail to prop it up to the correct height. I rarely fret this drone string, so it's okay that it's not over the neck its whole length.