Outlook does not directly expose the right-click context menu in its
CommandBars collection. You will, however, see a new right-click command is
when the item selected is using a custom form that includes one or more
custom actions. You can also add a custom action without using a custom
form, as demonstrated at http://www.outlookcode.com/codedetail.aspx?id=526

Richard Kagerer has posted a code sample at
http://www.outlookcode.com/codedetail.aspx?id=314 that shows how you might
trick Outlook into exposing the context menu through Explorer.CommandBars.

The C++ sample add-in at http://www.codeproject.com/atl/outlook2k3addin.asp 
also shows a technique for working with the context menu.