As the PP stated, simply book your package with a ticket length that reflects the number of days you want to spend in the parks. You do not have to add tickets equal to the number of nights/days you will be at the resort. You still need to eat, so if you want a dining plan, there's no harm in having that the day you check in.
If you were to book a room-only for one night and then a package the rest of your stay, you will end up with two reservations, having to check in twice and possibly having to change rooms. It just makes no sense to do this when you can adjust your tickets according to how you want to tour the parks.