If you salted your water properly in the beginning, you should not add salt when replacing the water that boiled away.
Water evaporates, salt does not. All the salt you put in the pot stays in the pot and the peanuts. As the water boils away the brine becomes more concentrated. Restoring the lost water is how you maintain the concentration of the brine that you started with.
Salt can be naturally extracted from sea water by allowing the water to evaporate, leaving the salt and other minerals behind.
For boiling peanuts, we suggest about 8 teaspoons of salt per gallon of water, which is 1 teaspoon of salt per pint of water.