IMHO, you shouldn’‘t change the server’‘s code. Your requirement will violate the XMPP spec. I think that the best way to go is to use a client that let’‘s you customize the popup dialog. I don’‘t know much about JBother but since it’'s built on top of Smack you may easily do that change. Besides, changing the server will imply that you will not be able to replace it easily since your branch will not be standard. Even more, you will have to update the server by your own when new versions come out.
Having said all that (big disclaimer) , if you still want to do that change on the server you will need to look at org.jivesoftware.messenger.roster.Roster#provideRosterItem(JID, String, List).