Your cramps could also be related to potassium deficiency. Cramping was an early on sign that I missed.
I am not sure about adding sodium etc. But I would be careful what type of salt you use in general. That is an issue I want to research for myself regarding general use of salt.
The use of supplements technically stops the fast ie. lemon juice, but would promote an extremely low calorie( known as starvation diet)alternative which is not as healthy for the body in as far as what I have learned about fasting.
Just for reference when you are not fasting , I heard Braggs vinegar with the mother is loaded with potassium as well as maple syrup. I still want to research Burrough's Lemonade Diet . Personally, I don't want the sugary drink. I think juicing would be more healthy is you are on a liquid fast. I guess I am an all or nothing sort of person, that is why I like water fasting, the kitchen cleanup is a breeze... :)