In your screen shot I see that you are using AUTO for the main level widths, this is going to create a menu that will not work in IE 6 or 7.
For compatibility with IE 6 and 7, the main menu items must use a pixel measurement for the width.
you can edit the menu items after the menu is created to add an inline width to give more granular control of each item, if you search this forum, you can find many examples of this.
to answer the original question: no the menus cannot be centered. however, you can use negative left margins for the sub menus to move them over to the left a bit so they dont sit directly under the left border of the parent menu item.