You should be able to add the DDP through the package you booked through dreams. I'd suggest contacting your Dreams agent directly--they would be more than happy to price it out for you, and add it if you want.
You can book the dining plan thru a TA. The real question is whether you have a package that will allow you to add the dining plan. Some packages that have significant room discounts cannot add the dining plan without losing the room discount (Disney is going to make their $ somewhere!).
If you call Dreams Unlimited, I'm sure they will give you the information on prices and add the dining plan if you desire.