I did this last year. They told me they didn't have the availability to add another day to FD but I could get a ro discount for one night....makes sense right? Anyways I called about once a month and checked daily for all my dates with no luck. Then I called after the FD offer booking period was over to ask a question about something and the cm said "I can combine this to one reservation with
free dining, you would just owe the extra to pay rack rate" I don't understand the rhyme or reason of it all but if you need to do it this way, keep asking.