In this workbook module:

Private Sub Workbook_Activate()
Call ShortCutMenuModify
End Sub

Private Sub Workbook_Deactivate()
Call ShortCutMenuReset
End Sub

In standard module:

Public Function ShortCutMenuModify()
Dim cbBar As CommandBar
Dim lX As Long
On Error Resume Next
For lX = 1 To Application.CommandBars.Count
   If CommandBars(lX).Type = msoBarTypePopup And CommandBars(lX).BuiltIn =
True Then
       Set cbBar = Application.CommandBars(lX)
       With cbBar
           .Controls.Add Type:=msoControlButton, Before:=1
           .Controls(1).Caption = "Custom"
           .Controls(1).FaceId = 5828
           .Controls(1).OnAction = "RunCode"
           If .Controls.Count > 2 Then .Controls(2).BeginGroup = True
       End With
   End If
Next lX
On Error GoTo 0
End Function

Public Function ShortCutMenuReset()
Dim cmdBar As CommandBar
Dim lngX As Long
For lngX = 1 To Application.CommandBars.Count
   If CommandBars(lngX).Type = msoBarTypePopup And
CommandBars(lngX).BuiltIn = True Then CommandBars(lngX).Reset
Next lngX
End Function

At first I thought that maybe 5 menus were affected and therefore, it
appears 5 times, but I counted the items and there are 65 menus affected, so
this has nothing to do with it...

Please let me know if you want me to post my shortcut menu as well...