How do you treat a sun burn while still being in the sun? Will adding sunscreen help?

Avoid the sun. You must stop the burning in order to heal. A sunscreen will help protect you from further sun, but it is not very good protection. The key to heal is sun avoidance, not sunscreen. You will need to moisturize your burn and keep it out of the sun. In the future, use an spf of no more than 50 and one that contains a metal such as zinc or titatnium oxide and wear a wide-brimmed hat.
Get out of the sun. Once you've started to burn, there has already been more damage to the skin than is immediately apparent. Most importantly protect the skin from further sun exposure/damage. Get out of the sun, if impossible cover the burned and other exposed skin areas with clothing to prevent further sun damage. Additional sunscreen would be my last option but it is better than nothing.