There's probably too much change for your old cat. I'd add the extra box like you are doing but I wouldn't expect the cat to adjust to a new box on top of the old one. I'd give her her old box and let the other cats have the new ones. Or you could try mixing some of her old (picked through) litter with the new one. Definitely don't change litters at this time. Also, it may sound silly but make sure she has some calm, one-on-one time with you (or whoever she loves most) everyday. Preferably at the same time of day. I had a similar problem and my friend who studies cat behaviour suggested that. AND I'd try to make a special area for the cat where she can relax and just watch life around her. My cat liked the top of her cat tree and I put it off to the side where she can still see everyone. My cat really liked to hop down on one of the lower ledges and swat the dogs when they went by and then flee back to the top lol. I think it gave her a sense of power.