I wont promise about the room only, but think you can reserve it with a hold on your credit card...
It seems as if you can add the package on at anytime, but I know if you are wanting to add on the DDP that has to be done a minimum of 72 hours prior to check in. Packages booked within 45 days of your trip will need to be paid in full.
With a package deal you have to pay $200 per person upon making the reservation but you don't have to pay anything else until 6 weeks prior to your arrival date when the balance is due. We have used the packages the last two times we have gone and I make payments every payday and usually have it paid for before the six weeks final payment. It is a great way to do it and then you only have to save for the souveniers and food! Right now, I wouldn't do it any other way unless I planned a trip at the last minute.