Hello Nandu,

You would need to override the ExpansionIndicatorCellControl ControlTemplate and modify it to include your new UIElements. 

It would end up looking something like this:

<UserControl x:Class="SilverlightApplication36.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:ig="clr-namespace:Infragistics.Silverlight.Controls;assembly=Infragistics.Silverlight.XamWebGrid.v9.1"
 xmlns:igPrim="clr-namespace:Infragistics.Silverlight.Controls.Primitives;assembly=Infragistics.Silverlight.XamWebGrid.v9.1"
 xmlns:igBase="clr-namespace:Infragistics.Silverlight.Controls;assembly=Infragistics.Silverlight.v9.1"
 xmlns:vsm="clr-namespace:System.Windows;assembly=System.Windows"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480">
 <UserControl.Resources>

  <Style TargetType="igPrim:ExpansionIndicatorCellControl" x:Key="MyExpansionIndicator">
   <Setter Property="Template">
    <Setter.Value>
     <ControlTemplate TargetType="igPrim:ExpansionIndicatorCellControl">
      <Grid Background="Transparent">
       <vsm:VisualStateManager.VisualStateGroups>
        <vsm:VisualStateGroup x:Name="CommonStates">
         <vsm:VisualState x:Name="Normal" />
         <vsm:VisualState x:Name="MouseOver">
          <Storyboard >
           <ObjectAnimationUsingKeyFrames BeginTime="00:00:00" Duration="00:00:00.0010000" Storyboard.TargetName="rectangle" Storyboard.TargetProperty="Visibility">
            <DiscreteObjectKeyFrame KeyTime="00:00:00">
             <DiscreteObjectKeyFrame.Value>
              <Visibility>Visible</Visibility>
             </DiscreteObjectKeyFrame.Value>
            </DiscreteObjectKeyFrame>
           </ObjectAnimationUsingKeyFrames>
          </Storyboard>
         </vsm:VisualState>

         <vsm:VisualState x:Name="Alternate" ></vsm:VisualState>
        </vsm:VisualStateGroup>

       </vsm:VisualStateManager.VisualStateGroups>
       <Border x:Name="border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}"/>
       <Rectangle Fill="Black" Visibility="Collapsed" x:Name="rectangle" Margin="{TemplateBinding BorderThickness}"/>
       <ContentPresenter Margin="{TemplateBinding Padding}"  VerticalAlignment="{TemplateBinding VerticalContentAlignment}" d:LayoutOverrides="Width" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"/>
       <igBase:ExpansionIndicator x:Name="Indicator" Visibility="Collapsed"  Margin="{TemplateBinding Padding}"  VerticalAlignment="{TemplateBinding VerticalContentAlignment}" d:LayoutOverrides="Width" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Cursor="Hand">
        <igBase:ExpansionIndicator.Style>
         <Style TargetType="igBase:ExpansionIndicator">
          <Setter Property="Foreground" Value="Blue"/>
          <Setter Property="Width" Value="20"/>
          <Setter Property="Height" Value="20"/>
          <Setter Property="Template">
           <Setter.Value>
            <ControlTemplate TargetType="igBase:ExpansionIndicator">
             <Grid Background="Transparent">
              <vsm:VisualStateManager.VisualStateGroups>
               <vsm:VisualStateGroup x:Name="ExpansionStates">
                <vsm:VisualStateGroup.Transitions>
                 <vsm:VisualTransition From="Collapsed" To="Expanded" GeneratedDuration="00:00:00.2000000"></vsm:VisualTransition>
                 <vsm:VisualTransition From="Expanded" To="Collapsed" GeneratedDuration="00:00:00.2000000"></vsm:VisualTransition>
                </vsm:VisualStateGroup.Transitions>

                <vsm:VisualState x:Name="Expanded">
                 <Storyboard>
                  <ObjectAnimationUsingKeyFrames BeginTime="00:00:00" Storyboard.TargetName="Plus" Storyboard.TargetProperty="Visibility">
                   <DiscreteObjectKeyFrame KeyTime="00:00:00" Value="Collapsed"/>
                  </ObjectAnimationUsingKeyFrames>
                  <ObjectAnimationUsingKeyFrames BeginTime="00:00:00" Storyboard.TargetName="Minus" Storyboard.TargetProperty="Visibility">
                   <DiscreteObjectKeyFrame KeyTime="00:00:00" Value="Visible"/>
                  </ObjectAnimationUsingKeyFrames>
                 </Storyboard>
                </vsm:VisualState>
                <vsm:VisualState x:Name="Collapsed">
                 <Storyboard>
                  <ObjectAnimationUsingKeyFrames BeginTime="00:00:00" Storyboard.TargetName="Minus" Storyboard.TargetProperty="Visibility">
                   <DiscreteObjectKeyFrame KeyTime="00:00:00" Value="Collapsed"/>
                  </ObjectAnimationUsingKeyFrames>
                  <ObjectAnimationUsingKeyFrames BeginTime="00:00:00" Storyboard.TargetName="Plus" Storyboard.TargetProperty="Visibility">
                   <DiscreteObjectKeyFrame KeyTime="00:00:00" Value="Visible"/>
                  </ObjectAnimationUsingKeyFrames>
                 </Storyboard>
                </vsm:VisualState>
               </vsm:VisualStateGroup>
              </vsm:VisualStateManager.VisualStateGroups>
              <Grid x:Name="ExpansionIndicator" Background="#00000000" HorizontalAlignment="Center" VerticalAlignment="Center">
               <TextBlock x:Name="Plus" Text="+" />
               <TextBlock x:Name="Minus" Text="-" Visibility="Collapsed" />
              </Grid>
             </Grid>
            </ControlTemplate>
           </Setter.Value>
          </Setter>
         </Style>
        </igBase:ExpansionIndicator.Style>
       </igBase:ExpansionIndicator>

      </Grid>
     </ControlTemplate>
    </Setter.Value>
   </Setter>
  </Style>

 </UserControl.Resources>
  <Grid x:Name="LayoutRoot">
  <ig:XamWebGrid x:Name="grid">
   <ig:XamWebGrid.ExpansionIndicatorSettings>
    <ig:ExpansionIndicatorSettings Style="{StaticResource MyExpansionIndicator}"></ig:ExpansionIndicatorSettings>
   </ig:XamWebGrid.ExpansionIndicatorSettings>
   <ig:XamWebGrid.GroupBySettings>
    <ig:GroupBySettings AllowGroupByArea="Top">
     
    </ig:GroupBySettings>
   </ig:XamWebGrid.GroupBySettings>
  </ig:XamWebGrid>
 </Grid>
</UserControl>