Welcome to the MPV forums!
A static graphic of a stop-watch / time can be inserted easily by importing a picture from your iPhoto Library... but I think you want something different, right?
If you need video footage of a stop-watch counting up, then the only way I know to do this is by inserting the video footage into iMovie.
1. Take close-up video of a stop-watch for duration you need from 0:00:00 to 10:00:00 for example.
2. Import this into an Event in iMovie.
3. Crop the Video (to remove any borders)
4. Turn on "Advanced Tools" in iMovie 11's Preferences.
5. Drag the stopwatch clip on top of the clip in your project. From the pop-up menu choose "Picture in Picture".
6. In the Viewer resize and move the stopwatch clip as desired and add fades in the Project area.
(You can re-use the stop-watch clip on all of Project clips as needed).
Hope this helps, and let me know how you get on. Am curious to know how this method works for you.