The TvPlanIt components create an internal popup menu (FDefaultPopup) at runtime - see the TDayView/TMonthView/TWeekView procedures InitializeDefaultPopup. Unfortunately this method is not public and not virtual, thus you cannot override it.
Of course you can drop a TPopupMenu component on the form and assign it to the dayview's/monthview's/weekview's PopupMenu property. Then, however, you must code the behaviour of this menu yourself, there is no way that you can reuse items of the built-in menu. Yet another design flaw of the original VisualPlanIt components...