How to add a blank row in datagrid in silverlight 4

<UserControl x:Class="SilverlightApplication52.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400" xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk">

    <Grid x:Name="LayoutRoot" Background="White">
        <sdk:DataGrid AutoGenerateColumns="False" Height="176" HorizontalAlignment="Left" Margin="50,35,0,0" Name="DataGrid1" VerticalAlignment="Top" Width="312" >
            <sdk:DataGrid.Columns>
                <sdk:DataGridTextColumn x:Name="FirstName"
                                        Header="First Name"
                                        Binding="{Binding Path=FirstName}"
                                        IsReadOnly="False"
                                        Width="150"/>
                <sdk:DataGridTextColumn x:Name="LastName"
                                        Header="Last Name"
                                        Binding="{Binding Path=LastName}"
                                        IsReadOnly="False"
                                         Width="150"/>

                <sdk:DataGridTextColumn x:Name="Contactid"
                                            Header="Contact Id"
                                            Binding="{Binding Path= Contactid}"
                                            IsReadOnly="True"
                                            Width="150"
                                            Visibility="Collapsed"/>
            </sdk:DataGrid.Columns>
        </sdk:DataGrid>
        <Button Content="New" Height="23" HorizontalAlignment="Left" Margin="23,242,0,0" Name="btn_new" VerticalAlignment="Top" Width="75" Click="btn_new_Click" />
        <Button Content="Save" Height="23" HorizontalAlignment="Left" Margin="114,242,0,0" Name="btn_save" VerticalAlignment="Top" Width="75" Click="btn_save_Click" />
        <Button Content="Delete" Height="23" HorizontalAlignment="Left" Margin="207,242,0,0" Name="btn_delete" VerticalAlignment="Top" Width="75" Click="btn_delete_Click" />
    </Grid>
</UserControl>