The promedias are an integrated set, the only way you can do it, is if your amplifier has a pre-out stage on the back, and simply not connect your Klipsch speakers to the sub. Some Yamahas did have this. You Would use a female 3.5" to Male RCA connector found at radio shack. (you will possibly have to use more than one to get the desired connection)
There are only two problems I see...
First is the processing speed of the onboard amplifier and the klipsch amplifier might be different, therefore you could get a delay (faster, or slower) in sub's response.
Second, the promedia's crossover is built around a small, higher frequency speaker. You will probably end up with gaps in the sound range. You can fix some of this with your Yamaha's crossover, but you will never be able to totally bypass the Klipsch crossover.
Edit. There are a few other ways that COULD work, but nothing is going to be the same as a separate dedicated sub.