=""""&G6&""""
will do it but not for a cell in situ.
So otherwise a macro:
[vba]Sub blah()
For Each cll In Selection.Cells
cll.Value = """" & cll.Value & """"
Next cll
End Sub
[/vba]will do this for the currently selected range. If there are any formulae in the selection, they will be converted to values.
p54cal