The only thing that I have found do to for this kind of thing is this:
<DataTemplate x:Key="BaseClass">
<!-- base class template here -->
</DataTemplate>
<DataTemplate DataType="{x:Type app:BaseClass}">
<ContentPresenter Content="{Binding}"
ContentTemplate="{StaticResource BaseClass}"/>
</DataTemplate>
<DataTemplate DataType="{x:Type app:DerivedClass}">
<StackPanel>
<ContentPresenter Content="{Binding}"
ContentTemplate="{StaticResource BaseClass}"/>
<!-- derived class extra template here -->
</StackPanel>
</DataTemplate>
Basically this creates a "common" template that can be referenced using a key (BaseClass in this case). Then we define the real DataTemplate for the base class, and any derived classes. The derived class template would then add it's own "stuff".
There was some discussion about this on msdn a while back, but no one came up with a better solution that I saw.
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