Adapter une fonction de J. Boisgontier...

Private Sub Worksheet_Change(ByVal Target As Range)
 If Target.Column >= 2 And Target.Column <= 14 And Target.Count = 1 Then
   Application.EnableEvents = False
   If [mémo] <> "" Then
     If Left([mémo], 1) <> "=" Then
        Target.Formula = "=" & [mémo] & "+" & Target
     Else
        Target.Formula = [mémo] & "+" & Target
     End If
   End If
   Application.EnableEvents = True
 End If
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
  If Target.Column >= 2 And Target.Column <= 14 And Target.Count = 1 Then
    ActiveWorkbook.Names.Add Name:="mémo", RefersToR1C1:="=" & Chr(34) & Target.Formula & Chr(34)
  End If
End Sub