how to add removed partition?
The following seems to work in Mysql 5.5.


ALTER TABLE `t_keyword_conversion_ga` REORGANIZE PARTITION p201106  INTO (
    PARTITION p201105 VALUES LESS THAN ('2011-05-01') ENGINE = InnoDB,
    PARTITION p201106 VALUES LESS THAN ('2011-06-01') ENGINE = InnoDB
);


as you can check by sqlfiddle