My understanding is that they can be added to your room keys.. without any problem.. should not matter if you have used a day or not.. nor do you have to purchase anything else to add to them.
HOWEVER..
You can't add dining "when you get there" nor can you add dining without first purchasing a package that includes at least one day base tickets..
Dining must be added to your reservation at least 3 days before arrival and again, ONLY as part of a package reservation that includes at least one day base tickets.
Hope this helps,
Knox