Yes you can add it up to a certain point. I can't remember how many days before, up to maybe 5 or so? Just call them and they will be happy to help you add it or answer the question of how many days before you have to have it added by.
If you would like to register, we have an entire board here dedicated to the Disney Dining Plan and I am sure they can answer this for sure and any other questions you might have, as well as help you to plan your meals!
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