If you want to add a row somewhere in between and not just at the end, you can edit the view of the webpage. Go to "Website > Configuration > Pages" and click on the page you want to change. Then click on the view. Here you can edit the source code of the page, including adding table rows. Be cautious, though, as this is not just HTML. Odoo uses QWeb as template engine and you might want to learn about this first.