Yes, on the (Actions) tab of a custom form. You can also add a custom action
without using a custom form, as demonstrated at
http://www.outlookcode.com/codedetail.aspx?id=526

Signature
Sue Mosher, Outlook MVP
Author of
Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
>I would like to extend Outlook with custom action that would appear on
> context menu of items (items in Inbox and possibly other folders).
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>
> Thanks.