If you modify the reservation prior to check in, but after the paid in full date for packages, there should be a fee for the extra adult, for their ticket, and for the DDP (everyone has to have all the same components, so if you bought tickets for three, you will now be buying tickets for four also). If doing this online, they should ask for payment at the time of modification. It should show you a page similar to when you first booked, showing the new total and requesting you pay at that time.
If you make the modification prior to your package paid in full date, they should add the additional amount you now owe to your original balance, and the entire amount would be due by your original pay in full deadline. For example, if you have $300 left to pay on your original package, and you add an additional $500 for the fourth person, your balance due would be $800 and due the same date as the $300 you had left.