Hi there, Trisha, and welcome back to planDisney!
I'm so happy to be able to help with this question because I get to share some good news - yes, you can add that
Park Hopper later in the day! The caveats are that it is subject to availability, so there is a chance the park could be at capacity depending on when you are visiting, and it does need to be within the operating hours of the other park (Disneyland Park, in your example, which is often open a bit later than Disney California Adventure Park). As long as that park has capacity and is still open, then you can purchase that Park Hopper addition later in the evening after you leave the first park you're visiting.
So the only question left is: when are you going?! This will be an absolutely magical year to visit the Happiest Place on Earth with the
Disney100 celebration and the
grand reopening of Mickey's Toontown! I've heard some great things about the new nighttime spectaculars as well -
World of Color - ONE and
Wondrous Journeys - and I'm so excited that the
"Magic Happens" Parade will be back starting February 24!
Thanks again for your question, Trisha! Come back and see us as you think of any other follow-up questions while you plan your visit to the Disneyland Resort.
Have a Magical Day,
Greg