This request has come up a few times, mainly for night flying. Normal bookings are by picking time slots, but instructors can now make bookings outside their working hours by clicking make a manual booking.
The system respects each instructor's working hours (which they maintain), and will not allow an instructor to be booked when they are unavailable. But if the instructor makes their own booking, this bypasses the check and they can book at any time.
Note: The booking appears on the List view but won't appear on the Day view if the booking falls outside the hours you've set for your normal booking times.