The XAML. Use null to define a separator.

<Menu Name="MainMenu" Grid.Row="0" IsMainMenu="True" ItemsSource="{Binding Model}">
    <Menu.Resources>
        <ControlTemplate x:Key="MenuSeparatorTemplate">
            <Separator />
        </ControlTemplate>
    </Menu.Resources>
    <Menu.ItemContainerStyle>
        <Style TargetType="{x:Type MenuItem}">
            <Setter Property="Command" Value="{Binding Command}" />
            <Setter Property="CommandParameter" Value="{Binding CommandParameter}" />
            <Setter Property="Header" Value="{Binding Header}" />
            <Setter Property="ItemsSource" Value="{Binding Children}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding }" Value="{x:Null}">
                    <Setter Property="Template" Value="{StaticResource MenuSeparatorTemplate}" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Menu.ItemContainerStyle>
</Menu>