That's a good way to have a call that has two distinct pitches, so you can easily sound like more than one duck. Just drill about a 1/4" hole about 2/3 of the way to the end of the insert. Make sure to use a fast drill press with a good, sharp bit or it could catch and hang up, which could crack your call.
The Mallard Militia: We kill more ducks before 7:00 am than most people kill all day!!