The context menus you can't get at. Even in Outlook 2007 you can only get at
the Explorer context menus.

When the Inspector opens for an item you can access its CommandBars
collection. Item.CommandBars.Item("Standard") give you a reference to the
Standard toolbar. You can get its Controls collection and use the Add method
to add a button to that toolbar. If you use the Add method of the
CommandBars collection you can add your own toolbar.

Sample code for that is available but without knowing your development
language I can't specify a specific link. You can review the information at
www.outlookcode.com for samples in various languages.