I've done something similar using layers in Dreamweaver. I created a layer for each of the menus, which are hidden when the page loads. Then I made one layer with a huge transparent gif that covers the entire nav area and about half inch past it on each side. I set that layer to make all other layers disappear when the mouse passes over it. That way the menus disappear when the person moves out of the nav area. Then set all of the other layers to appear on mouseover and the other layers to hide. It really didn't take that long and it works very well.
There are some programs out there that automate the process but I've tried a few and the menus worked great alone, but as soon as I put them into the page they didn't work right anymore. GRRR!! That's very frustrating. If you use Dreamweaver and can figure out how to use layers, you can do quite a bit with the nav bar (and other things too!).
I hope this is of some help to you!