Hi Lehua! Thanks for visiting planDisney.
You should be able to add
Park Hopper to your ticket either before you arrive on a valid unused ticket or after having some fun at Disneyland Park on your park day. I can quickly walk you through how to go about these two choices.
If you'd like to add Park Hopper ahead of time, please call the
Tickets phone line. A Cast Member can assist you in adding this great option. Convenient methods of payment include credit, debit, and Disney Gift Card. Be ready with your ticket information and the MyDisney account e-mail you used to purchase your tickets. You can also attempt to do this yourself. Log into your MyDisney account and hover over "My Disneyland" on the Disneyland Resort website. Select "Change Your Theme Park Tickets" from the dropdown menu. Select the tickets you'd like to change and follow the onscreen prompts.
If you'd like to wait and see if you'd like to add Park Hopper after you've arrived and spent some time in Disneyland Park, you'll simply need to stop by the main ticket booths between the parks and speak with a Cast Member when you're ready to hop. They should be able to add Park Hopper to your ticket. You'll need to pay the difference. We like to hop after lunch, but you can hop over to Disney California Adventure Park as early as 11:00 AM Pacific Time with Park Hopper added to your ticket. You can even hop back to Disneyland Park! It's super flexible and lets you explore as much as your heart desires.
I hope you have a magical time.
Elizabeth