Yes, you can use the Key Color (Transparency) video transition to impose a video over another. Add your first video to Video 1 of the timeline. Add the video that you want to impose over the first video to Video 2. Then under the Video Transitiont track, add the Key Color transition to the Transition track of the timeline. Finally, from Transition Properties, select the color that you want to make transparent (in your case, the color Green).