WPF Expander control dropdown panel - wpf

Question

I would like to style my expander control in such way that it can have a drop down panel like that of combobox control.
Drop down panel should be like it can overlay above other items like that of combobox.
Here's the XAML style for expander which i'm using :
<Style TargetType="{x:Type Expander}">
<Setter Property="Background" Value="{StaticResource Brush_ButtonFill}"/>
<Setter Property="BorderBrush" Value="{StaticResource Brush_ContainerButtonBorder}"/>
<Setter Property="Foreground" Value="{StaticResource Brush_FontReadonly}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*" x:Name="ContentRow"/>
</Grid.RowDefinitions>
<Border Visibility="Collapsed" Grid.Row="1" x:Name="ExpandSite" Background="{StaticResource BrushTransparent}" BorderBrush="{StaticResource Brush_DropdownShadow}">
<ContentPresenter Focusable="false"/>
</Border>
<Border Grid.Row="0" x:Name="Border" Background="{StaticResource Brush_ButtonBorder}" Visibility="Collapsed" BorderThickness="1"/>
<ToggleButton Template="{StaticResource ExpanderToggleButtonControlTemplate}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" OverridesDefaultStyle="True" Background="{StaticResource Brush_ButtonFill}" BorderBrush="{StaticResource Brush_ButtonBorder}" BorderThickness="{TemplateBinding BorderThickness}" Foreground="{StaticResource Brush_ButtonBorder}" d:LayoutOverrides="VerticalAlignment" />
<ContentPresenter Margin="15,2,2,2" RecognizesAccessKey="True" ContentSource="Header" VerticalAlignment="Center" HorizontalAlignment="Left" IsHitTestVisible="False"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="True">
<Setter Property="Visibility" Value="Visible" TargetName="ExpandSite"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Opacity" Value="0.5"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ExpanderStyle_Dropdown" TargetType="{x:Type Expander}" BasedOn="{StaticResource TextStyle_BaseControl}">
<Setter Property="Background" Value="{StaticResource Brush_ButtonFill}"/>
<Setter Property="BorderBrush" Value="{StaticResource Brush_ButtonBorder}"/>
<Setter Property="Foreground" Value="Black"/>
<!--<Setter Property="Foreground" Value="{StaticResource Brush_FontReadonly}"/>-->
<Setter Property="FontSize" Value="{StaticResource FontSizeMedium}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="54"/>
<RowDefinition Height="*" x:Name="ContentRow"/>
</Grid.RowDefinitions>
<Border Visibility="Collapsed" Grid.Row="1" x:Name="ExpandSite" Background="{StaticResource Brush_ButtonShadow}" CornerRadius="0,0,5,5" Margin="0,-5,0,0" Panel.ZIndex="1000">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Focusable="false" Margin="5,10,5,5" Panel.ZIndex="1000"/>
</Border>
<Border Grid.Row="0" x:Name="Border" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="7" Padding="5" Background="{StaticResource Brush_CZBlueSelected}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="50"/>
</Grid.ColumnDefinitions>
<Border Grid.Row="0" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="5,0,0,5" Padding="5" Grid.ColumnSpan="1" Margin="0,0,1,0"/>
<Border x:Name="Shadow" CornerRadius="5" Padding="5" Grid.ColumnSpan="2" BorderBrush="{StaticResource Brush_SearchinputBorder}" BorderThickness="0,0,0,2" Margin="0,0,0,-2"/>
<ToggleButton Template="{StaticResource ExpanderToggleButtonControlTemplate_DropDown}" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" OverridesDefaultStyle="True" Background="{StaticResource Brush_ButtonFill}" BorderBrush="{StaticResource Brush_ButtonBorder}" Grid.Column="1" BorderThickness="{TemplateBinding BorderThickness}" Foreground="{StaticResource Brush_ButtonBorder}" Margin="1,0,0,0" Height="44" />
<ContentPresenter Margin="4" RecognizesAccessKey="True" ContentSource="Header" VerticalAlignment="Center" HorizontalAlignment="Center" IsHitTestVisible="False"/>
</Grid>
</Border>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="True">
<Setter Property="Visibility" Value="Visible" TargetName="ExpandSite"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Opacity" Value="0.5"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Now using the same in XAML screen as:
<Expander Name="expanderNextScan" Header="{Binding ElementName=listBoxExpander,Path=SelectedItem.Content}" ExpandDirection="Up" Grid.Column="3" Height="54" Width="Auto" Style="{DynamicResource ExpanderStyle_Dropdown}" >
<Grid Background="#FFE5E5E5" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
<ListBox x:Name="listBoxExpander" Height="100" Width="{Binding ElementName=expanderNextScan,Path=ActualWidth}" ScrollViewer.HorizontalScrollBarVisibility="Auto" ScrollViewer.VerticalScrollBarVisibility="Visible" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch" Margin="0,-10,0,0" Panel.ZIndex="5">
<ListBoxItem Content="Go to Next Scan" FontSize="{StaticResource FontSizeMedium}" IsSelected="True"/>
<ListBoxItem FontSize="{StaticResource FontSizeMedium}" Content="Print with preview" />
<ListBoxItem FontSize="{StaticResource FontSizeMedium}" Content="Export as PDF"/>
<ListBoxItem FontSize="{StaticResource FontSizeMedium}" Content="Export to DICOM"/>
</ListBox>
</Grid>
</Expander>
Now want to style it in such a way that the list box coming in the expander drop down should have a z-index greater than the other controls on the screen such that it can overlay over other controls like that of combobox dropdown.
Please reply ASAP.