Yes, an admin can schedule appointments on the back end whenever they'd like.
To schedule an appointment outside of your availability, either click directly on the calendar or click on the +Appointment button at the top of the Appointment Calendar to schedule an appointment.
After selecting the type of appointment (and calendar, if applicable) you can click on the Custom button to select a date and time outside of the availability that you have set.
Finish scheduling the appointment as you normally would. Just a heads up, you will get a warning that reads "The time conflicts with another appointment or is outside your normal hours". If you click through this warning, the appointment will be scheduled.