The original question was:
Is there a formula to convert pounds of thrust into horsepower?
The simple answer is "no", unless you also specify the speed.
Power = force x velocity ( x unit conversion factor).
Several posters have read the question the other way round:
"is there a formula to convert horsepower into pounds of thrust?".
Again, the simple answer is "no".
Thrust = power x propellor efficiency ( x unit conversion factor).
Since propellor efficiency is not something you can put in a formula, out come the graphs.
But as earlier posters have suggested, there are some rule-of-thumb formulae that will give you a rough approximation for a given case.
And sorry, but I'm not getting out my sliderule to do all the unit conversions. It's probably all in wikipedia, anyway.
Kalispera sas, Jimmy !