SELECT @right_most_sibling := rgt
   FROM Personnel
  WHERE emp = ? /* :your_boss */;

 UPDATE Personnel
    SET lft = IF(lft >  @right_most_sibling, lft + 2, lft),
        rgt = IF(rgt >= @right_most_sibling, rgt + 2, rgt)
  WHERE rgt >= @right_most_sibling;

 INSERT INTO Personnel (emp, lft, rgt)
 VALUES ('New Guy', @right_most_sibling, @right_most_sibling + 1);