You can't is the short answer. As far as I can see you can only manipulate the Z Axis on some of the 3D Transitions. Clearly this will just apply to the transitions rather than a video clip itself.
Having said that, you can apply a clip over a solid colour so that the transition is almost as long as the clip itself. then manipulate the transition on the Z AXIS.
Enjoy
____________________ Yougle - The number one rated Movie, Video & Film search engine to ever emerge!