Yes its possible, only if I'm right looking at the connector you'll only be able to use the l&r speakers, as the input socket accepts l&r and sub seperately, ie the cross over is in the main unit.
You could connect l&r direct to the input, then link off the l&r inputs with 1 k resistors to the sub input to create a mono channel, while this would work you'd get full range from the sub so sound quality wont be as good as it could, plus you'd get a little less stereo effect from summing l&r with the resistors.