its actually nothing to do with conversion at all.
a joule is equal to (mass*distance^2)/time^2[seconds].
hp = kg*g[gravity](9.8m/s^2)*distance/seconds
now doing simple conversion to american (std) and you get
hp=lbs*g*ft/sec
or in poundsforce hp=lbf*ft/sec where lbf=lbs*9.8 m/s^2(gravity)
now rather than going in a discussion about this, just read wikipedia's defination of torque in relation to hp (since i have gotten the units to measure hp for you)
http://en.wikipedia.org/wiki/Torque