追加できるのはCanvasに対してだけ？

どこにでも追加できるわけではない。

書くとこんな感じ

                      <Canvas Height="25" Margin="119,0,0,46.5" VerticalAlignment="Bottom" HorizontalAlignment="Left" Width="175">
                            <Canvas.Triggers>
                                <EventTrigger RoutedEvent="FrameworkElement.Loaded">
                                    <BeginStoryboard>
                                        <Storyboard>
                                            <DoubleAnimation Storyboard.TargetName="rect8" By="40" 
                                                    Storyboard.TargetProperty="(Canvas.Left)" Duration="0:0:1.2" RepeatBehavior="Forever"/>
                                            <DoubleAnimation Storyboard.TargetName="rect9" By="40" 
                                                    Storyboard.TargetProperty="(Canvas.Left)" Duration="0:0:1.2" RepeatBehavior="Forever"/>
                                            <DoubleAnimation Storyboard.TargetName="rect10" By="40" 
                                                    Storyboard.TargetProperty="(Canvas.Left)" Duration="0:0:1.2" RepeatBehavior="Forever"/>
                                            <DoubleAnimation Storyboard.TargetName="rect11" By="40" 
                                                    Storyboard.TargetProperty="(Canvas.Left)" Duration="0:0:1.2" RepeatBehavior="Forever"/>
                                        </Storyboard>
                                    </BeginStoryboard>
                                </EventTrigger>
                            </Canvas.Triggers>

                            <Rectangle Height="20" Stroke="Black" StrokeThickness="2" Canvas.Left="0" Canvas.Top="0" Width="150"/>
                            <Rectangle x:Name="rect8" Height="20" StrokeThickness="2" Width="20" Fill="#FF686868" Canvas.Left="0" Canvas.Top="0"/>
                            <Rectangle x:Name="rect9" Height="20" StrokeThickness="2" Width="20" Fill="#FF686868" Canvas.Left="40" Canvas.Top="0"/>
                            <Rectangle x:Name="rect10" Height="20" StrokeThickness="2" Width="20" Fill="#FF686868" Canvas.Left="80" Canvas.Top="0"/>
                            <Rectangle x:Name="rect11" Height="20" StrokeThickness="2" Width="20" Fill="#FF686868" Canvas.Left="120" Canvas.Top="0"/>
                            <Rectangle x:Name="rect7_Copy1" Height="20" StrokeThickness="2" Width="20" Fill="White" Canvas.Left="145" Canvas.Top="0"/>
                        </Canvas>