If you want to add items to the various menus, go to Preferences | Change Calibre Behavior and look at the Toolbar option.
Just remember that some of the menus you see (Main toolbar, and Menubar) have different settings for when there is no device attached and when there IS a device attached.
Which ever menu you are modifying, just drag icons for the tools / plugins you want to see from the left hand (available actions) to the right hand side (current actions).
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