• Hacking to make ALTER TABLE online for certain changes 
• ALTER TABLE without locking the table? 
• ALTER TABLE performance with MyISAM, or MySQL: Loading large tables with Unique Keys

 To add a column to a very large MyISAM table, here are the steps that is proofed to work fine:

1. Create a new table of the same structure, including the new column, without keys
2. Dump the data in the old table into a csv file:
   % mysqldump -u user -p -t -T /tmp --fields-enclosed-by=\" --fields-terminated-by=, database table
   (You may not need the option --fields-enclosed-by)
3.  If you need to add data to the csv file, use a script to do so
4. Load the csv file into the new table:
   mysql> LOAD DATA LOCAL INFILE '/tmp/table.txt' INTO TABLE newtable (col1,col2,..);
5. Create another table with all keys defined
6. Copy over .frm and .MYI files:
   # cp newtable2.frm newtable.frm
   # cp newtable2.MYI newtable.MYI
7. Flush the table:
   mysql> FLUSH TABLE newtable;
   (Login as root or as a use that has the "reload" grant)
8. Repair the table:
   mysql> REPAIR TABLE newtable QUICK;
9. Drop the old table:
   mysql> DROP TABLE oldtable;
10. Rename the new table back to the old table:
    mysql> RENAME TABLE newtable TO oldtable;

That's it.