You want to make your talks mandatory for your attendees.
You can make sure all talks are added to your attendees' schedule when they sign up by enabling auto-registration of your attendees in your basic settings.
Find out how to do this in this help doc - https://help.heysummit.com/help/you-want-to-auto-register-your-attendees.
You can select the sessions added to attendees' schedules when importing.
Find out how to do this in this help doc - https://help.heysummit.com/help/you-want-to-import-your-attendees-e8a08ae8