Extending Composite Structure model generally/in Enterprise Architect

Question

I'm currently on a project, which integrates domain specific techniques in the modelling tool Enterprise Architect through UML Profiles.

The domain specfic model almost matches the structure of the composite structure model, with a couple of restrictions though, so I'm trying to use the model as a foundation for my restrictions.

The questions:

1) How can I generally extend models (like composite structure models) which themselves lie in the M1-layer of Meta-Object Facility?

2) Is the element "Part" of composite structure diagrams a metaclass itself, which can be extended by stereotypes in UML profiles?

Since the literature on these topics is not very exhaustive, I'm also looking for suggestions, too.