First of all, please excuse me for my _bad_ english

I need to set up a horizontal menu with vertical submenu. It works fine but if the menu is located and the end of the page, the submenu shows itself below the main menu. I want it upside.
I've searched through menu documentation, menu options (all of them) and the forum, but I didn't find a clue.
Note: my web site is still in developpement, and can't be accessed online. To understand clearly what I mean, please go here:
http://milonic.com/menusample23.php
On the Top Offset field, enter "2800" and check the "center" radio button. At the end of the page, you will see the horizontal menu which shows up the submenu below itself, and hide a part of the submenu. Can I setup the menu to have the same thing with the submenu upside, and make itself fully visible?
Thank you in advance
G.Estival