Adding raw salt to a pot cooking on the fire or Blech on Shabbat constitutes Bishul (cooking) and is prohibited. The Bet Yosef (Siman 318) cites Rabbenu Simcha who deals with a case in which a personally intentionally violated this Halacha and added salt to a food on the fire. He rules that if there was no salt whatsoever in the food from before Shabbat, then the food becomes forbidden to consume. The salt adds a prohibited taste to the entire dish. However, if there was already some salt in the dish, but he merely added more to enhance the flavor, then it is permissible to eat the food. Such a case in which the flavor is comprised both of a permissible source and a forbidden source is known as "Zeh V’zeh Gorem" and is permitted. The Rema in Siman 318 rules in accordance with Rabbenu Simcha.
SUMMARY
If one intentionally added salt to a food cooking on the Blech on Shabbat, the food may not be eaten, if there was no other salt added from before Shabbat. However, if the food already had salt, but he merely added more, it is permitted, after the fact, to consume the food.
