Hi, Jamie!
I love that you are already planning to hop! And yes, Disney does allow to add the
Park Hopper option at any time during the course of the duration of your ticket. The cost is a flat fee per ticket, no matter what time of day you choose to upgrade, meaning if you choose to hop in the morning, you'll pay the same fee as if you upgraded at 7:00PM.
Assuming you do not have Florida Resident ID, the cost of the Hopper upgrade on a single-day, single-Park ticket will be $35.00 ($37.28 with tax) for each Guest, regardless of age.
You can upgrade your ticket at any Guest Relations counter or at any ticket window.
Happy hopping!
See you later, celebrator!
-Deborah B.