Can we customize the MessageComposer

I want that based of the role of the user, some users should need approval from a supervisor.
So if the role is probation instead of the send button they should see a "request approval" button

and that button should set the email to draft so when the supervisor sees it can send it if correct.

the question is about modifying the MessageComposer to shows this different buttons.
the functionality i can do with a handler.

Thank you