If I read your post correctly, this code should do it. If a footer
exists already, then this code adds to it. Note this Usenet interface
word wraps, so you may need to revise the line breaks.

Sub Append_Footer()
Dim Sh As Worksheet

For Each Sh In Sheets
With Sh.PageSetup
If .CenterFooter < "" Then .CenterFooter = .CenterFooter & " " &
Chr(10) & "Some more text"
End With
Next Sh

End Sub