[Sparkz]

This is how I did the exact same thing in the installer of my Default PM-hack.

SELECT @displayorder:=displayorder+1 FROM settinggroup ORDER BY displayorder DESC LIMIT 1;
INSERT INTO settinggroup (title, displayorder) VALUES ('Default PM', @displayorder);
SELECT @settinggroupid:=LAST_INSERT_ID();
INSERT INTO setting (settinggroupid, title, varname, value, description, optioncode, displayorder)
       VALUES (@settinggroupid, 'Send default PM to new users?', 'defpmactive', '0', '', 'yesno', 1);