Room only and packages are sold by separate sub divisions of the company. So, you can't convert a RO to a package. It requires a cancel and rebook. Which requires availability for the new reservation, which they may or may not have. If you cancel the RO first, the availability may not show up right away, or someone else could grab it, so you have to make the package reservation before canceling the RO.
However, even if there is no availability, if you call the reservation number, and tell them what you want to do, a supervisor could override the system (assuming you get someone with the knowledge and willingness), and essentially overbook the hotel, knowing they will be simultaneously canceling the first reservation. However, if the resort is already overbooked for some reason, they won't be able to do that.
Not sure if they can apply the funds directly from one to the other, or it would require a refund and a new payment.