I would start by assuming that a nylon guitar string sold to tune to a B would be a likely choice to use for a B on a nylon strung banjo. Any fault in my logic, Craig?
Following my first assumption, I would make a set from the first four guitar strings, plus an extra first, to use a banjo 5th string. I would then listen to how it sounds/plays, and revise my theorem accordingly.
Quack!
Geoff the Duck.