Even though this question was asked 4 years ago, I'll answer it! (Good ol' search function!)
The John Guest valve fits most 1/4" airline/icemaker line so you can concievably add any CA reactor to it if that is the size of it's effluent line. The idea here is that the effluent will still have CO2 in it that makes the effluent low ph (acidic), so the foaming action of the skimmer will help "blow off" some of the dissolved CO2 before it gets back into the tank.
The paradox is that (I've heard) skimmers "skim" better if the water is more alkaline. So putting a kalk reactor effluent line in that same hole should make the skimmer itself work better! However, my experience with my kalk reactor says that the valve will soon be encrusted in calcium deposits if you do that.
Anyway, that's why the valve is there. I hear that adding Ozone to the skimmer at that point will help it skim better as well.
V