This is how I used to solve this task:
$('select').kendoDropDownList({
open: function(e) {
e.sender.popup.element.addClass('test');
}
});
I was not able to find any configuration way.
Demo: http://jsfiddle.net/n9V4N/2/
Or another way to do it:
var dropDown = $('select').kendoDropDownList().data("kendoDropDownList");
dropDown.list.addClass('test');