Customizing Entourage

Is it possible to add a new button or menu item to the new message
window in Entourage 2004 ?
Alternatively is it possible to hook into
the attach file button / drag and drop event for this window ?
I want to provide a way of selecting files/folders which if they meet
certain criteria are either allowed to be attached in the normal
manner to the new message or if not they are moved to an appliance and a link to
the attachment is inserted into the message before sending it instead
of attaching the actual file.