Couple of options for you. I’d recommend making a template using the page.php from your theme. Look around here to find out how to do that if you’re not sure. Right before the end of the Loop, add the following code <?php if (comments_open()) comments_template(); ?>. Use this template for the Submit Themes page.
Of course, you can add this code to your existing page.php file and be done with it AFTER you make a backup copy of your page.php file in case something goes wrong.