You can just add it on to your existing cruise if there is still availability. But I'm pretty sure you cannot book either with ABD, that you have to do it through DCL. The phone number on that page goes to DCL.I have not added the ABD cruise add on just yet - with everything going with the virus right now - I am holding off as I am not sure I want to tie up $$ with ABD right now...
I did have a question though - I have the cruise booked already through DCL - can I just add on the ABD cruise add-on to my existing cruise reservation through ABD or do I need to cancel my existing reservation through DCL and book both the cruise and ABD cruise Add-on together through ABD?