First of all the menu items must be there, so you'll have to change how the main menu is rendered on the server side. Maybe you can do it with a PageTree control instead of the PageList which it probably uses OOTB.

Then you have to do the dropdown stuff on the client side. You should be able to do this with CSS only, but there are also many javascript solutions. Google for css drop down menu or something and you'll get thousands of hits. Each strategy will require you to output the HTML in a format which matches that solution, which of course determines how you need to wire things up on the server side to render the menu html.

Removed: Accidental double-post.

Thanks Magnus!