You can always go ahead and add it--but if you add it through the skimmer it's only going to dissolve while the pump is on. They do have a liquid version of CYA if you want to dissolve it in a hurry, but I understand it's quite a bit more expensive. You might have time to dissolve some before you leave by hanging in old sock or stocking in front of the return and giving it a squeeze now and then to speed it up.

If you're able to maintain chlorine overnight but not during the day, it's reasonable to assume you have zero, or at least too little, CYA in the pool.