how to add background video to an exisiting video

Yes. Use the Picture in Picture feature to overlay the existing video onto the background video, and adjust the size of the existing video so that it is a little smaller in dimensions than the background video that it is overlaying.

So, put the background video into the main timeline and put the existing video into the timeline above it. Then select the existing video by clicking on it. Then click on the overlay icon in the toolbar in the top of your screen (the one that looks like overlapping squares) and choose Picture in Picture in the little selection box that appears. The existing video will appear on your screen inserted into the background video. Drag the corners of of the inserted video to adjust its size as necessary.

— Rich

The overlay icon should appear when you have selected the clip in the upper timeline that you want to insert. It will be outlined in yellow.