How to add separator to ComboBox items

<ComboBox HorizontalAlignment="Left" Margin="66,82,0,0" VerticalAlignment="Top" Width="120">
            <ComboBoxItem>Administrator</ComboBoxItem>
            <Separator></Separator>
            <ComboBoxItem>Junior Clerk</ComboBoxItem>
            <Separator></Separator>
            <ComboBoxItem>Senior Clerk</ComboBoxItem>
            <Separator></Separator>
            <ComboBoxItem>Manager</ComboBoxItem>
        </ComboBox>