You do not have to book everything inside Viamente. Reservations made elsewhere can be added to your journey so flights, stays and other details remain part of the same context.
Adding an existing booking helps keep the itinerary coherent and gives you one place to see what is booked, what is planned and what still needs attention.
Viamente does not automatically change or cancel an external reservation. For changes, refunds or provider-specific questions, use the original booking provider.