I'm still not sure what you're asking. If you've worked with WP in the past, then you should already know that indenting an item (drag & drop) in the menu editor will change it to a sub-menu (dropdown). If that's not what you're trying to do, then I must not be understanding your explanation.
Ok cool, I've got dropdown menus that expand, nice. The next thing I'd like to work out is where I edit the link that the menu goes to. I've attached a snip of where I mean.