it should be the jiveid and the jiveroster table you need to modify. But as Wildfire assumes that it’‘s the only database user it caches the rosters etc.it’'s not a good idea to use SQL. The recommended way is to write a plugin and use the Wildfire API to modify the roster.
I usually don’'t write Wildfire Plugins so I can just jive you the hint to read the Plugin Developer Guide (no idea where one can find it now) and to post in the Wildfire Dev forum to ask questions regarding coding or development.