Hi there, Emilie, and welcome to planDisney!
Unfortunately, payment plans are only able to be added during the purchase process for
Magic Key Passes. I did
call a Cast Member at the Magic Key Pass line, and she confirmed that the prompt to enroll in a payment plan (only available for California residents) occurs during the initial purchase process on the Magic Key Pass purchase page. The Cast Member I spoke with also confirmed they do not have the ability to change to a payment plan after that initial purchase process. I'd imagine that's not the answer you were hoping for - so sorry about that!
However, I'm hopeful that you will have a year of amazing trips to the Disneyland Resort ahead of you! They sell out so quickly, so I'm glad you had the opportunity to purchase one. While my family and I haven't had a chance to purchase Magic Key Passes, many of my fellow planDisney panelists have them, so I have been able to vicariously live through them on their frequent visits to the parks - especially on their visits to the
Magic Key Terrace!
Thank you again for your question, Emilie. I hope you have a year of fun with your new Magic Key Pass!
Have a Magical Day,
Greg