1) create a field that it's not shown in the form of the frontend
2) the field could be mofified in the backend by administrator
3) the field and the value must be shown on the attendees list
Example : i have a text field that contain information about the status of registration and the field value can be "REGISTERED" or "PENDING".
The visitor sign at our event but can't see this field in the form. As administrator in the backend i can change the value of this field in "REGISTERED" or "PENDING".
In the attendees list the registrant can see this information with other field.