Hello Lucien,
I haven't used SEOMenu so can't comment on that, you may be able to achieve what you want with the HouseMenu however.
Check out
www.skinningtoolkit.com which implements the HouseMenu. - Click on Main Classes, you will see additional menu items appear at the bottom of the menu.
In order to set the menu up as you desire it will take a bit of configuration though, if you follow this tutorial it will provide the information you need to adapt the menu.
(7 videos – 65mins)
Watch the introduction video (3.8MB)
You will probably need to set up a skin with different house menu configurations for each root menu item.
You may also find it useful to download the simple red leaf CSS skin as that provides a vertical menu which you could use for the starting point of styling the menu.
Free Skin - Simple Red Leaf CSS SkinIt will take some time to work it out as I have not implemented the menu in this way, so if you can find another menu that does it instantly then it will probably save you a lot of time. - But these links above should give you enough information to get started and experiment with the configuration.
Thanks,