You're actually almost spot on.Run PVC pipe to furnace exhaust vent.....you can pull outside air from there into skimmer....sorry, I assume its a gas furnace, you have a skimmer and you wont be using the downstairs furnace, and its on its own exhaust......just a thought....
Furnace room is actually where I produce RO, store foods in a mini freezer/fridge.. It's on the other side of the wall, it's actually not a crazy idea.
I'm pretty sure there is a cold air intake that goes outside, I didn't even think of that.