Create a new video line above the video track you want as a background. Then create a new title and place it on this new line where you want it. You can specify which bits you want to have transparent, and by how much, when the title editor is open.
Then just dissolve from the video to the title, and when you're happy with the length, dissolve back to the video.
Throughout this the video will be visible in the parts which you chose to be transparent or left blank.