It's only possible to add a thumbnail from a video's still frame.
Navigate to your individual video’s Edit Media screen (Media Library –> click the title of the video of interest). Play a video, wait till you find a desirable frame, then click “Capture Thumbnail” button. The video will pause and display “uploading…”. Then it will display “Thumbnail saved”. Click the “OK” button and your thumbnail image (also known as a poster frame) is updated automatically.
Also, don't forget the ideas forum, where you can discuss your suggestion with the WordPress.com community: http://en.forums.wordpress.com/forum/ideas