It can be very frustrating...I seemed to get a different answer from everyone that I talked to at RCCL. The last person I talked to before I went on my cruise said "Yes--you can arrange for them to add the tips on your Credit Card when you check-in at the pier". Of course, when I checked in at the pier, they said "No, you cannot do that!".
Maybe if enough people add this issue to the Comment Cards at the end of the cruise, they could resolve it once and for all!