You can remove the jumpers (don't lose them as you may want them in the future) from the preamp out to the main in (see page E-7 of your Yamaha manual) and put in Y adapters, splitting from the preamp output with one end going to the main in and the other to the "line in" on the subwoofer. You will need to do both to get the bass from both channels to the subwoofer.
Also, pay attention to what is stated in your Yamaha manual on page E-7 regarding PURE DIRECT and CD DIRECT AMP.
The result of this will not be ideal, because you will still be sending a full range signal to your main speakers. For proper matching, you will want to select a crossover for the subwoofer that is at the bottom of the range your main speakers can do. Otherwise, you will get both the main speakers and the subwoofer reproducing the sound that the bottom of your main speakers reproduce, resulting in boomy bass (though some people like that).
