Welcome to planDisney, Shelley. We're so glad you're here to share this question with us. It won't be long until you will be walking down Main Street, U.S.A. towards Sleeping Beauty Castle and I'm here to help you prepare.
Unfortunately, there isn't a way to add a
Mickey Mouse Celebration Cake to an existing reservation in the
Disneyland app or at the site. If there are still
dining reservations available for the restaurant where you will be eating, I recommend making a new reservation and adding the celebration cake to your new reservation. Keep in mind that this cake is only available at
select dining locations. If your reservation is at a particularly popular spot (like Blue Bayou Restaurant), I don't recommend canceling and trying to make a new reservation as the likelihood of success is very slim. Instead, there is the option to order a cake in person at the restaurant's host podium on the day of your reservation. There are only a limited number of cakes available, so I recommend heading to the restaurant early in the day to make your request for your best chance.
It sounds like you will be celebrating something special, so be sure to pick up a complimentary "I'm Celebrating:" button from one of the shops on Main Street, U.S.A.
Halloween Time at the Disneyland Resort is also in full swing and there's no shortage of spooky things to enjoy. You can check out the digital guide in the Disneyland app so you don't miss a thing. Snapping a photo with the iconic giant Mickey pumpkin is always a must for me every spooky season.
Shelley, while I wish I had a better answer to share, I hope the information I shared is still helpful. Please come back to visit us with any other questions. Here's wishing you a magical time at the Happiest Place on Earth!
Michelle