It is allowed to extend stays of rooms in this option: Front Office → Reception → Modify Registration and selecting the new departure date in the Departure field.
Only if the stay is extended, the system will verify that there is availability for the room type and that the room is available for the new dates.
In case you wish to extend the stay even if there is no availability, the operation must be performed by a user with a level equal or higher than the one authorized in the parameter: Rcp_Level_Ext_Avail.