I might be misunderstanding your question, but you don't need to use binding to put an instance of one type of control into an instance of another type of control. I'd recommend making someKindOfControl derive from ContentControl, then you could do this:
<DataTemplate>
<someKindOfControl>
<bbb/>
</someKindOfControl>
</DataTemplate>
Just make sure you use a ContentPresenter in your default style for someKindOfControl - that'll determine where bbb shows up.
On the other hand, if you have many controls that you want to insert into someKindOfControl you'd be best off using template parts to insert the controls and providing a style for someKindOfControl within the DataTemplate:
<DataTemplate>
<someKindOfControl Style={StaticResource SomeKindOfStyle}/>
<DataTemplate>
Where SomeKindOfStyle provides a ControlTemplate that puts many types of custom control into various template parts of someKindOfControl:
<UserControl.Resources>
<Style x:Name="SomeKindOfStyle" TargetType="myNamespace:someKindOfControl">
<Setter Property="ControlTemplate">
<Setter.Value>
<ControlTemplate>
<bbb x:Name="PART_TopRightControl/>
<bbb x:Name="PART_BottomLeftControl/>
<bbb x:Name="PART_CenterControl/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</UserControl.Resources>
That's a bit more complicated but can allow you to put multiple custom bbb controls into one instance of someKindOfControl. Keep in mind I'm not totally sure what you're after, but if you post a bit more info I might be able to clarify.