Since you don't know what going internally with her I wouldn't add anything to her water, plain water is good. If you have some pop bottles you could fill them with warm water put a blanket over them and keep her as near to them as possible. Nutritionally I am very inexperienced with what chickens need when they are unwell and not pooping. I would imagine that if there is some pullet feed or gritty high vitamin feed at hand that you can mix in with the egg yolk that may help for sustainability. Otherwise at this point it is the waiting game, and that is the hardest part until the vet can see her.