Do you have a multi-day ticket? Or a single day ticket?
The park hopper option can technically be added to your ticket at the ticket booths before you enter the parks. You could do this the day before or if you’re local, you could go do it now, but you can only do it at the ticket booths.
If you have a multi-day ticket, the park hopper is applied to all days. It’s one price whether you have a one-day ticket or a five-day ticket (obviously there is more value with a multi-day ticket). You cannot add a park hopper to only one day of a multi-day ticket.
All that said, you can only add a park hopper to your existing ticket IF park hopper reservations are available for that day. If you’re trying to upgrade to a park hopper on the same day you plan to enter the parks, and park hopper reservations are sold out for that day, you will be denied, regardless of the reservation you already hold for that day. It’s very difficult to add a park hopper day off, especially if it’s a busy time when reservations are sold out.