You can add it still. I think the cutoff for those types of changes is 48 hours before you arrive. And if you make a change within 10 days of your arrival there is supposed to be a charge. However, when I upgraded our DDP from QS to regular DDP a few days before a trip they didn't charge me for that change (I figured that was because I was paying more money to them, rather than making a change that would cause me to spend less money).
If you don't already have them, I would make sure you have reservations to use up your TS credits before paying to add the dining plan though. With only a week to go, you may find reservations quite difficult to get if you don't already have some.
Even though you may not want to make an international call, the only way to do this is by telephone. Can't be done online, and you must make any changes to your package no later than 72 hours before arrival. Dining plans can't be added at check-in. If you have a travel agent, contact them ASAP if you want the dining plan added.
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