Can I add/change the number of hash subpartition?

[Osamu Shibata]

I have a table with partitions and subpartitions.
Structure is

CREATE TABLE `test` (
`col1` tinyint(2) unsigned NOT NULL DEFAULT '0',
`col2` date NOT NULL,
PRIMARY KEY (`col1`,`col2`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
/*!50100 PARTITION BY RANGE (to_days(col2))
SUBPARTITION BY HASH (col1)
SUBPARTITIONS 10
(PARTITION p2008 VALUES LESS THAN (733773) ENGINE = MyISAM,
PARTITION p2009 VALUES LESS THAN (734138) ENGINE = MyISAM,
PARTITION p2010 VALUES LESS THAN (734503) ENGINE = MyISAM,
PARTITION p2011 VALUES LESS THAN (734868) ENGINE = MyISAM,
PARTITION pmax VALUES LESS THAN MAXVALUE ENGINE = MyISAM) */;

It works well.
After that, I want to add subpartition. The number of subpartitions from 10 to 12.

For partitions,
"ALTER TABLE test ADD PARTITION PARTITIONS 2;"
this is the correct query but what about subpartitions?

I tried some like
"add subpartitions...."
"reorganize partitions .... subpartitions 12"
All didn't work.

Maybe remove partitioning and add partition again to unpartitioned data?