Are you talking about having it on screen while other footage plays on top of it? I don't think you can do this, but am just getting familiar with iMovie 09 myself. I bought the book "iMovie 09 The Missing Manual" which I would recommend as a great resource. I know you can do PiP (Picture in Picture) which would let you have the scoreboard in the picture in picture screen. Also, you can put clips of the scoreboard in between frames periodically to update people on the score changing. Hope that helps.
Peace 🙂