You could use
http://www.gmayor.com/document_batch_processes.htm to handle the documents and run the following as a custom process:
Code:
Function AddFooter(oDoc As Document) As Boolean
On Error GoTo err_Handler
Dim oSection As Section
Dim oFooter As HeaderFooter
For Each oSection In oDoc.Sections
For Each oFooter In oSection.Footers
If oFooter.Exists Then
oFooter.Range.Text = "Confidential - Not for distribution"
End If
Next oFooter
Next oSection
AddFooter = True
lbl_Exit:
Exit Function
err_Handler:
AddFooter = False
Resume lbl_Exit
End Function