Carbonated water and the byproduct of baking soda reacting with an acid both off gas carbon dioxide. Mixing carbonated water with lemonade will produce slightly fizzy lemonade. Adding baking soda to lemonade will just release the gas into the atmosphere unless you do it in a closed container like a soda bottle which is potentially extremely unsafe if you don’t know how much baking soda to add. Adding baking soda in a closed container may carbonate the lemonade because the CO2 dissolves into solution instead of escaping, but it’s also likely to explode if the container can’t withstand the pressure or has a defect.