Hi,
Well, I don't use any of those xtras menus but from looking at the menu.xml file that looks like where you code your style and your menus. Overflow=scroll is a menu property, that is a property that gets declared in the menu not the style or items so following the example I'd try adding
and see if that worked. That's how the other properties seem to be declared, i.e.
and so on. I'm assuming here that the xml menu version can use the same properties as the other menus so you'd put them in the area to which they belong, that is in the style section, the menu head section, or an item.
Let us know if this doesn't work.
Ruth