If you are talking about getting FREE dining, you can only get that if you book the whole package through Disney. If you want to pay for the dining plan you can probably add it but I'm not sure how it works with Expedia?
Generally, if you book an Expedia package, or an Expedia allocation with a room-only reservation, then no, you aren't eligible for the Dining Plan, free or paid.
The Dining Plan requires purchase of a Magic Your Way package. They're available from Disney directly, or from AAA. Travel agents can often make Magic Your Way package reservations, but sometimes prefer not to, because such reservations don't come from the travel agent's allocation.