Sheldon converted an IRO frame w/ similar horizontal drops to multi-speed using a Shimano Nexus hub:
I suggest you just put a bigger chainring and skinnier tires on your p1, or trade it for something else....or maybe just buy a beater mountain road bike used for $50 and use that for commuting. Save the P1 for urban / trials / off road / whatever you originally bought it for.
The derailleur I took off my conversion was an old shimano 5/6 speed, it actually bolted on the wheel axle, so it could be used on any bike. Of course, then you have to route cables. Just get another bike for commuting. It will be cheaper in the long run.