Your idea using md5 is actually the simplest way I know of. ILIAS converts the password hash to bcrypt on the next login of the user.
This query should do the trick (set 'xxxxxxxxx' to your desired password):
UPDATE usr_data SET passwd = md5('xxxxxxxxx'), passwd_enc_type = 'md5' WHERE usr_id = 6;
This query should do the trick (set 'xxxxxxxxx' to your desired password):
UPDATE usr_data SET passwd = md5('xxxxxxxxx'), passwd_enc_type = 'md5' WHERE usr_id = 6;