Inclua a seguinte macro de evento na área de código da planilha:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim r As Range
Application.EnableEvents = False
For Each r In Target
v = r.Value
If InStr(1, v, "X") > 0 Then
r.Replace "X", " X "
r.Replace " X ", " X "
End If
Next r
Application.EnableEvents = True
End Sub