Nancy, Yes, you can add salt anytime while boiling peanuts. We think it is is better to add salt later rather than earlier for many reasons.
Some people say raw peanuts boil quicker when boiled without salt. This has also been noted with dried beans, which are legumes like peanuts.
Peanuts do not take up the salty water until late boiling, thus many people add too much salt before the peanuts are wet and fully saturated with the brine.
Add salt later after they are soft and let the boiled peanuts soak in the salt water as they simmer or cool.
Letting boiled peanut soak in the salty water will insure full saturation of the salty flavor.