Co2 will brake your WE G18C sooner than later no matter what upgrades you make, starting with the noozle.
It will also most likely make it useless for cqb.
Also Co2 is pretty inconsistent by itself.
But if you really want to do it just get Co2 mags like Sewdhull said.
Or get a pistol with a plastic slide (TM) if your problem is gasinconistency in coolder weather.