You can certainly insert the username, time and date into a document using VBA, though it would have no security value.
Code:
Sub InsertNameAndDate()
Selection.TypeText Environ("UserName") & Chr(32) & Chr(150) & Chr(32) & _
Format(Date, "Long Date") & Chr(32) & Chr(150) & Chr(32) & _
Format(Time, "HH:MM")
lbl_Exit:
Exit Sub
End Sub
You could include a scanned signature graphic, but that would not improve security either, and you would need to arrange a selection of images one for each user.
This is not a substitute for digitally signing a document.