No, you won't be able to add the DDP. Expedia does not sell packages that include the DDP and they technically "own' the reservation until you check in. You will not be able to call Disney and "do" anything to your reservation if you make it through a 3rd-party (travel agent, AAA, whatever, not just Expedia).
If you want the DDP, you must make your reservation through a service that offers it, like a full-service travel agent (AAA, others) or through Disney itself.
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