It is straightforward and relies on the use of a range
Code:
Sub Pluriel()
'Declare the variables used
Dim oRng As Range
Dim lngCase As Long
'set a range to the word the cursor is in
Set oRng = Selection.Words(1)
'establish the case of the word
lngCase = oRng.Case
'if the word ends in a space or non-breaking space remove that space from the range
oRng.MoveEndWhile Chr(32) & Chr(160), wdBackward
'Collapse the range to its end
oRng.Collapse 0
If lngCase = 1 Then 'its upper case
oRng.Text = "S" ' write S to the range
Else 'its not upper case
oRng.Text = "s" 'write s to the range
End If
lbl_Exit: 'set a marker - not essential
Set oRng = Nothing 'clear the range
Exit Sub 'and finish
End Sub