Hi Natasha,
That’s not possible without modifying one of the existing templates.
Modifying a template is not simple, because it requires at least a working knowledge of PHP, HTML, CSS and possibly JavaScript, but with good coding skills you can fully customize the appearance of your menus.
As a workaround, you could try to split your subcategories on more levels, and that should sensibly shorten your single-column sub-menus.