scroll box submenu

Hi. I have the classified theme and under -main menu I have a submenu with a bunch of submenus
Main
Sub
sub
sub
sub
etc, down to about 50 now but need to increase it by about 30 more at least.
these subs are all about the subject of the first sub menu, that’s right under the main.
How would I make all these sub-sub menus instead of dropping down in a long line, drop down in a box with a scroll bar say about 10 subs in the box. The way it is now, it offers a very very crappy use to scroll a mile to the bottom.
Thanx very much, John