If your speaker cable is 2-conductor, then your only choice is to send a mixed mono speaker output to your single speaker. If you had run a 4-conductor cable, you could install a variety of dual voice coil single speakers that have stereo inputs.
As your Russound controller/amplifier does not appear to have the ability to output mono as a per-zone setting, something like the Audioplex Stereo-to-Mono Mixer would be your only option with a 2-conductor speaker cable run. The only thing is you have to be certain that whatever device you choose does not use a common ground, as tying the speaker grounds together would damage the digital amplifier channels in the controller.
Steve