Do a search for "Hampton Bay" in the Pronto, Pronto NG and X10 forums and you will find it likely that no one has succeeded in doing this.

You need to either
- modify your Pronto so that it transmits RF at 303.875 MHz instead of 418
- build an IR to RF converter which takes IR from your Pronto and changes the carrier frequency to 303.875MHz.

Then you have to find a way to get your codes into your Pronto.