Catch theme free : menu horizontal

Hi everybody, I post today because i want to do a simple thing but I can't found it in the forum. I have the classic horizontal primary menu, and when i open a category, it scroll down, that's OK. My problem is : i dont want "accordion" submenu for my "main works" category (http://didier-hebert-guillon.com) because it is divided by years. To be more understandable, i want, when the user open the "main works", it open the years horizontaly (something like the main menu), and after, when mouse over on a year, have the classic scroll down as usual. I expect i am understandable, sorry for my english, i'm french :) Thanks for your answer !!