Why do you want to blind oncoming traffic with fog lights that are energized when you have low beam selected? If you switch from high beam to low beam to reduce glare to oncoming traffic, having the low beams enable fog lights seems Sooo counterproductive. Fog lights, generally, are un-aimed. The light goes everywhere. Not good.
I think both high and low beams will be operated by a relay, not the actual switch. That said, you could add another relay or tap into the relay output as a source or trigger. I would not do that, presonaly, and would rather a separate switch to enable fog lights. In that case, I would add wiring, a switch, and a relay to operate the lights. Make sure to fuse whatever you add, and be sure that it is de-energized when you turn the ignition to OFF.
Am I missing something..
Fog lamps are intended to provide an adjunct to the low beams. Because fog hovers close to the ground, the lamps are designed to shine down, illuminating the road beneath the fog. The top of the beam is cut off sharply so the light does not shine into the fog and reflect off it.