Unfortunately, we are unable to add additional travelers over 2 years old to existing bookings. If you'd like to add an additional traveler to an existing booking, you'll need to book a separate ticket for the passenger.
You can either:
Book a ticket with the same airline and itinerary through the Hopper app.
Contact the airline directly with the flight numbers and purchase an additional ticket directly through them. They might be able to link the 2 bookings.