The only exception is guests staying at a DVC resort on points. You can be a DVC owner or a guest "renting points". Guests staying at a DVC resort that booked through CRO has the same package requirements as other resorts.
Unfortunately, as Lewis pointed out, if you're not staying at a DVC resort on DVC points, admission purchase is required in order to be eligible to purchase the Dining Plan.
All you need to purchase is a one day ticket for each person in order to be able to purchase the dining plan. The ticket can be used at a later date, it may be worth it to you?