There is a fundamental Halachic principle which states, "Keli Rishon Mebashel," which means that a utensil in which food had been cooked over a fire has the capacity to cook raw foods, even after it is removed from the fire. It is thus forbidden on Shabbat to add raw food to a pot of food that had been on the fire, so long as the food is at or above the temperature of "Yad Soledet Bo" (the point at which ones hand would immediately recoil on contact). Since the food is in its original pot, and it is still hot, it is capable of cooking raw food that is poured into it, which would constitute a Shabbat violation.
One common example of this Halacha involves seasoning foods after they are removed from the fire. It occasionally happens that a woman removes a pot of rice from the stove on Friday night in preparation for serving, and then realizes that she had forgotten to season the rice. Even though the pot is no longer on the stove, it is forbidden to add raw spices such as pepper and cumin, since these spices would be considered as being cooked by being poured into the pot of hot rice. Instead, the woman should transfer the rice from the pot to a serving bowl, and then add the spices. According to many opinions, food in a Keli Sheni ("second utensil," as opposed to the original utensil in which the food was cooked) does not have the ability to effectuate "cooking." It is therefore permissible to add raw spices to hot rice after it has been removed from the pot in which it had been cooked.
It should be noted that one may add salt even to the original pot of rice, if the pot has been removed from the fire. The salt that we purchase today has undergone processing which entails cooking, and pouring it into a Keli Rishon thus does not constitute "cooking." It is therefore permissible to add salt to a hot rice even before it is transferred out of the pot. Nevertheless, one may not add salt to a pot over the fire, despite the fact that salt has already been cooked. It is only after the pot is removed from the fire that one may add salt.
Summary: It is forbidden to add raw spices to a pot of food, even after it has been taken off the fire, if the food is still hot. Spices may be added only after the food has cooled, or after it has been transferred out of the pot into a serving bowl. Salt, however, may be added to a pot of hot food, provided that the pot has been removed from the fire.
