I presume you are talking about export to mp4 video. (Not sure what you mean by overlay in Moving Elements.) In any event, I know of no way to extend the video in Photoshop Elements. However, once you create the mp4 video file, you could use Premiere Elements (or any other robust video editor) to extend the time of the video.
I've actually made a nice cristmas video just by copying and looping that 6 second clip in Priemeire elements (and windows movie maker- a very old version that still works). I created the scene in daz3d. I created multiple copies of that 6 second loop, then added the msuic onto a separate track. Basically the samething Greg said. I usually post videos like thatto my youtube channel so they look better.