Yep, it has nothing to do with tickets, you just have to be a resort guest. They will take your money for the dining plan not problem. I would caution you to run the numbers though. It hasn't been a good deal in over a decade and now that they cut even more and raised the prices, it is a horrible deal.
If you now have an AP you have to be a WDW resort guest. Have you been a Dis guest that has dined there a lot? If that’s the case you may do better cost wise by dining out of pocket and being choosy about what you order. We’ve done the DDP on a lot of trips and being wise as to how to utilize it we’ve saved money in the past, but it’s never been a great amount of savings. It all depends on how wise you are in your dining.
Yes you can purchase the dining plan.
But how you put it saying "if they offer the dining plan" makes me think you're wondering about if they offer a FREE dining plan. If that's the case you would need to do what the offer requires which is usually a minimum hotel and ticket package.