Martin, You can't do that.
Grid is a type of panel, with content as children property. So, if you add anything to it in XAML designer it will be relpaced.
However you can override childern property, and add this to your class <ContentProperty("PropertyName")>
like in the example -
Ex:
'Code:
<ContentProperty("Children")> _
Public Class MyGrid
Public Overloads ReadOnly Property Children As UIElementCollection
Get
Return Me.ContentGrid.Children
End Get
End Property
End Class
'Markup
<Grid x:Class="MyGrid" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<Button Content="Button" Height="23" HorizontalAlignment="Center" VerticalAlignment="Center" Width="75" />
<Button Content="Button" Height="23" HorizontalAlignment="Center" VerticalAlignment="Center" Width="75" Grid.Row="2" />
<Button Content="Button" Height="23" HorizontalAlignment="Center" VerticalAlignment="Center" Width="75" Grid.Row="1" />
<Grid Name="ContentGrid" Grid.RowSpan="3"></Grid>
</Grid>