Bootstrap 3 is still at RC, but I was just trying to implement it. I couldn't figure out how to put a sub menu class. Even there is no class in css and even the new docs don't say anything about it
It was there in 2.x with class name as dropdown-submenu
I bumped with this issue a few days ago. I tried many solutions and none really worked for me on the end i ended up creating an extenion/override of the dropdown code of bootstrap. It is a copy of the original code with changes to the closeMenus function.
I think it is a good solution since it doesn't affects the core classes of bootstrap js.
You can check it out on gihub: https://github.com/djokodonev/bootstrap-multilevel-dropdown