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I may be missing somethin’, but I’m wondering whether you can create a new blueprint component type based on an existing component type, ie: myboxcomponent extends boxcomponent would be a boxcomponent with some extra thing in its graph?
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In general, yes. For box components specifically? Not yet. We’re kind of ‘soft-launching’ the Blueprintable components functionality, so we marked only UActorComponent and USceneComponent as Blueprintable for 4.7. In future releases we’ll test and roll out the ability to BP subclass more types of components (including making sure they’ve got a good BP API to work with). I just marked UCameraComponent as Blueprintable for 4.8 for example 
You can mark any of your own custom C++ components with Blueprintable in their UCLASS() declaration if you want to be able to subclass them in the editor though.
Cheers,