The (formerly) accepted answer by Honza seems incorrect, see its comments. It seems not possible to use a GRANT query to alter multiple users at once since MySQL does not support wildcards for user names.
As you suggested yourself you can alter records in the mysql.user table directly using an UPDATE query and as Marc Delisle suggested, afterwards flush priviliges with:
FLUSH PRIVILEGES;
Also see dba.stackexchange.com > How to grant multiple users privileges.