TabControlStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <!-- Полностью скопированный стандартный шаблон TabControl -->
    <!-- за исключением панели переключения элементов -->
    <Style x:Key="ControlStyleWithAddButton" TargetType="{x:Type TabControl}">
        <Setter Property="Padding" Value="2"/>
        <Setter Property="HorizontalContentAlignment" Value="Center"/>
        <Setter Property="VerticalContentAlignment" Value="Center"/>
        <Setter Property="Background" Value="White"/>
        <Setter Property="BorderBrush" Value="#FFACACAC"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type TabControl}">
                    <Grid x:Name="templateRoot" ClipToBounds="True" SnapsToDevicePixels="True" KeyboardNavigation.TabNavigation="Local">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition x:Name="ColumnDefinition0"/>
                            <ColumnDefinition x:Name="ColumnDefinition1" Width="0"/>
                        </Grid.ColumnDefinitions>
                        <Grid.RowDefinitions>
                            <RowDefinition x:Name="RowDefinition0" Height="Auto"/>
                            <RowDefinition x:Name="RowDefinition1" Height="*"/>
                        </Grid.RowDefinitions>

                        <!-- Панель переключения элементов: начало -->

                        <!-- Стандартная панель переключения -->
                        <!-- <TabPanel x:Name="headerPanel" Background="Transparent" IsItemsHost="true" Margin="2,2,2,0" Grid.Row="0" Grid.Column="0" KeyboardNavigation.TabIndex="1" Panel.ZIndex="1"/> -->

                        <Grid Grid.Column="0" Grid.Row="0">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="Auto"/>
                                <ColumnDefinition Width="*"/>
                            </Grid.ColumnDefinitions>

                            <Button Grid.Column="0" 
                                    Width="30" Margin="10,0,2,0" Padding="0" 
                                    Command="{Binding RelativeSource={RelativeSource AncestorType=Window, Mode=FindAncestor}, Path=DataContext.AddCommand}" 
                                    CommandParameter="{Binding CommandParameter}">

                                <TextBlock Text="+" FontWeight="Bold" FontSize="22" Margin="0" Padding="0"/>
                            </Button>
                            <TabPanel Grid.Column="1" x:Name="HeaderPanel" Background="Transparent" IsItemsHost="True" Margin="2,2,10,0" KeyboardNavigation.TabIndex="1" Panel.ZIndex="1"/>
                        </Grid>

                        <!-- Панель переключения элементов: конец -->

                        <Border x:Name="ContentPanel" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.Column="0" KeyboardNavigation.DirectionalNavigation="Contained" Grid.Row="1" KeyboardNavigation.TabIndex="2" KeyboardNavigation.TabNavigation="Local">
                            <ContentPresenter x:Name="PART_SelectedContentHost" ContentTemplate="{TemplateBinding SelectedContentTemplate}" Content="{TemplateBinding SelectedContent}" ContentStringFormat="{TemplateBinding SelectedContentStringFormat}" ContentSource="SelectedContent" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                        </Border>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="TabStripPlacement" Value="Bottom">
                            <Setter Property="Grid.Row" TargetName="HeaderPanel" Value="1"/>
                            <Setter Property="Grid.Row" TargetName="ContentPanel" Value="0"/>
                            <Setter Property="Height" TargetName="RowDefinition0" Value="*"/>
                            <Setter Property="Height" TargetName="RowDefinition1" Value="Auto"/>
                            <Setter Property="Margin" TargetName="HeaderPanel" Value="2,0,2,2"/>
                        </Trigger>
                        <Trigger Property="TabStripPlacement" Value="Left">
                            <Setter Property="Grid.Row" TargetName="HeaderPanel" Value="0"/>
                            <Setter Property="Grid.Row" TargetName="ContentPanel" Value="0"/>
                            <Setter Property="Grid.Column" TargetName="HeaderPanel" Value="0"/>
                            <Setter Property="Grid.Column" TargetName="ContentPanel" Value="1"/>
                            <Setter Property="Width" TargetName="ColumnDefinition0" Value="Auto"/>
                            <Setter Property="Width" TargetName="ColumnDefinition1" Value="*"/>
                            <Setter Property="Height" TargetName="RowDefinition0" Value="*"/>
                            <Setter Property="Height" TargetName="RowDefinition1" Value="0"/>
                            <Setter Property="Margin" TargetName="HeaderPanel" Value="2,2,0,2"/>
                        </Trigger>
                        <Trigger Property="TabStripPlacement" Value="Right">
                            <Setter Property="Grid.Row" TargetName="HeaderPanel" Value="0"/>
                            <Setter Property="Grid.Row" TargetName="ContentPanel" Value="0"/>
                            <Setter Property="Grid.Column" TargetName="HeaderPanel" Value="1"/>
                            <Setter Property="Grid.Column" TargetName="ContentPanel" Value="0"/>
                            <Setter Property="Width" TargetName="ColumnDefinition0" Value="*"/>
                            <Setter Property="Width" TargetName="ColumnDefinition1" Value="Auto"/>
                            <Setter Property="Height" TargetName="RowDefinition0" Value="*"/>
                            <Setter Property="Height" TargetName="RowDefinition1" Value="0"/>
                            <Setter Property="Margin" TargetName="HeaderPanel" Value="0,2,2,2"/>
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter Property="TextElement.Foreground" TargetName="templateRoot" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>