Yes you can, no problem! You just need to telephone your local Disneyland reservations centre – they'll be happy to help and you just pay the difference between what you paid when booking and the then current price of your new chosen package. Alternatively, if the extra guest has or can separately buy a park ticket (including an annual passport), you can simply add them by informing reception and paying the local tax which is a euro or two per night.
Do be aware that if you booked using a special offer which has expired when you change the booking, the entire holiday will be repriced to the then current rate.