Hey Sherrie,
Yes, you can. To do this, click Edit Stage on the Sermon Cue to begin editing what goes out to the Stage Display. Go to Insert Object to insert your Countdown Clock. Then, on the ribbon under the Home Tab, you can set it to count up (instead of Countdown). This will only display on the Stage Display because that's what screen you inserted it on.
I hope this helps!
Thanks,
-Ryan