Or even better still, if you are into originality or concourse examples in your collection, forget all of the above. Just cut the short screw in half, sit cut end in lathe, square off, centre and bore in, say 6mm or so with a 2mm or so drill. With me so far? Now take standard piece of similar o/d rod, cut to new required length, turn pin on each end. Fit together, silver solder, make good, clean up and paint and that's it. Simple, original, takes about 15 minutes or 20mins if you rush the job.
Works a treat, made myself a long Thompson front grip screw from a rear grip screw not a few months ago. It's called '....where there's a will there's a way'. Or in this case, 'where there's a will, there's a SIMPLE way.....'