While this can only be done indirectly at the moment, it is still relatively easy to set up. What you need to do is create a private rate and a secret voucher code tied to it.
As long as the guests are unaware of the existence of this voucher code, they will not enter it when making their bookings via the Booking Engine, and therefore the special rate will only be known (and bookable) by your property's staff.
Please see the below guide for instructions on setting up such a voucher:
And here you will find the guide for creating the private rate to attach it to: