public class CustomRadDropDownButtonElement : RadDropDownButtonElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadDropDownButtonElement);
        }
    }
    protected override RadDropDownButtonPopup CreateDropDown()
    {
        return new CustomDropDown(this);
    }
}

public class CustomDropDown : RadDropDownButtonPopup
{
    public CustomDropDown(RadElement ownerElement) : base(ownerElement)
    {
    }

    protected override void EnsureItemEnabled(RadItem item, bool isUp)
    {
        do
        {
            item = this.GetNextItem(item, !isUp);
            if (item == this.Items.Last())
            {
                break;
            }

        } while (!item.Enabled);
        if (item != null)
        {

            this.SelectItem(item);
        }
    }
}