For songs in A flat (1st fret) up to C (5th fret) you can capo the fifth string together with the lower four strings, using railroad spikes. The position of th spikes can vary, I have em at the 2nd, 4th and 5th fret, so A, B and C but for b flat I will capo the fifth string at the 2nd fret (A) and tune up the string half a note. I read somewhere that experienced banjo players won't capo at the fifth fret anyway because they will play songs in C without a capo, so in open G tuning.
Songs in D are played using the open G tuning, but with the fifth string either tuned up OR capoed at the 2nd fret. This will change the standard gDGBD into aDGBD. For songs in E and F you can then again tune up all strings equally.
Not sure what your plans are, but I have found several remarks that it is wisest to let spikes be installed by an expert, as this is a tricky thing to do correctly (position of the spike will affect the banjo, too tight/loose, sharp edges etc). Unless you are a very handy guy, of course.... In that case, I have a little bag with six spikes here plus there is a very clear YouTube vid about installing spikes !