If you're using the new tween, there's no explicit way to do it, but it can be simulated. If you're using classic tween, there's no way to do it at all.
The way to do it is to use a similar trick to how you get abrupt transitions in gradient fills-- you have to put two control points immediately next to each other, so the transition happens instantly. To do this in a tween, go into Refine mode, then for both X and Y click the Add Anchor Point button and place a new anchor immediately before the anchor point for the new scale. You'll probably have to fiddle with the curve handle to get a straight line to the left of the control point. To the right of the control point doesn't matter since the transition will be happening over a single frame.
For simple animations with only a single motion, you may also be able to achieve the same effect by applying a custom ease to the Scale component.
You could copy and paste the frames with your existing tweens into a movieclip, and then scale the movie clip down.