You can add a 2019 Season Dining Plan to your membership, but it's actually more expensive for Diamond Elite.
The Deluxe Dining Membership price for Diamond Elite Members is only $4.70 per month, which is only $56.40 per year.
However, while you did spend more money, you've successfully delayed the headache of trying to add a Dining Membership onto your existing Membership! Doing so would actually require you to cancel your existing membership and start a brand new membership, thus restarting the 12-month commitment contract.
How many people are you buying memberships for? This will affect what the best course of action since when you do eventually add a Dining Membership and restart your Membership, you'll have to get whatever the current deal is at that time. And normally the deal is that you must buy 2 or more memberships to get the lowest advertised price. So you may have to choose between overlap paying for Dining twice for a few months (since your pass would still be active), or paying a higher monthly membership fee.
Have a Six Flags day.