Adding custom fields to Joomla Component

We are using a Joomla component Event Booking which has a feature to add custom fields to each event, currently these fields are assigned to an event when creating the custom field. When creating a new event there are two sections: Billing Fields and Group Member Fields. This contains the core fields like: last name, address, city, etc. where the admin can assign which fields are shown for each event. The custom fields are not listed here. We want to add any custom fields to this page, so that when a new event is created, the admin can choose which fields will be used for that event.