The only way anti-freeze will keep water from freezing is to add the anti-freeze to the water, not only would this contaminate the entire potable water system, THIS WOULD BE DEADLY.
You might as well just drink the stuff (which is what you will be doing).
I stated this strongly so you will not attempt to do it.
Adding the anti-freeze to the upper bladder, if you found a way to get it into the air valve, would not keep it from freezing, and if the bladder fails and develops a leak (as they do from time to time) you will have just contaminated the potable water system as though you added the anti-freeze to the water directly.
Just to make sure you are clear on this – DO NOT DO IT.