plugin hooks on message headers table

[loudgefly]

Hi,
I'm trying to develop a plugin, and i need to modify the original ui, exactly, i wanto to add a row in the message header table in message preview.
I have  already read http://trac.roundcube.net/wiki/Doc_Plugins   http://trac.roundcube.net/wiki/Doc_Skins http://trac.roundcube.net/wiki/Doc_SkinML and the other documents, but i'm quite confused about what to do:  in which way i can add content to message header table? do i need to use a 'register_handler( <object_tag_name> , array($this, 'my_function'))' statement? in this case,  what string should i substitute for <object_tag_name>? I tried 'messageHeader' , but nope.
any help will be appreciated.
Thanks