If you're only adding one extra page, and it's an "under construction" page thats going to be there until the site is completed, the only thing that you need to do is upload the holding page as 'index.html'. Once you do that your web server will automatically display that file instead of your Wordpress home page. When you're ready, delete that file and the live site will be actually live.
The only drawback to this is that while you're devloping, all of the 'Home' links on your WP site will point to the 'under construction' page. All you need to do is go to http://www.mysite.com/index.php and you'll be able to see the Wordpress home page.