Dovecot quota via custom column

[Filip Bartmann]

Hello,
we have multiple domains connected to single user account and we want to have quota counted via this column - called account_id. Users are stored in mysql database.
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iterate_query = SELECT email AS user FROM mailboxes
user_query=SELECT email, 501 AS uid, 501 AS gid, CONCAT('*:storage=',quota,'M') AS quota_rule FROM mailboxes AS m WHERE email='%u'
password_query=SELECT email AS user, pw_hash AS password, CONCAT('*:storage=',quota,'M') AS userdb_quota_rule FROM mailboxes AS e WHERE email='%u'
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We have quota stored in dovecot dictionary and as usable variables we found only - http://wiki2.dovecot.org/Variables. 

Is there any way to do use account_id as quota username?

Thanks,
Filip Bartmann