Neither by the candidates, your internal team, nor by clients, can breaks be added retrospectively to timesheets. Furthermore, when updating their timesheet (if necessary) after checking out, candidates would not be able to edit the time they started their break. However they can update the length of their break. Your internal team can also update this information wherever they update timesheets.
Clients will be able to see the break time and hours, as indicated by the coffee cup icon, when they approve timesheets, but they will not be able to make any edits.