[Exim] Per virtual user quotas?

Hello all,

I am doing virtual domains with exim with the following:
virtual_localuser:
driver = aliasfile
transport = virtual_localdelivery
domains = lsearch;/mail/conf/domains
file = /mail/conf/$domain/passwd
search_type = lsearch

virtual_localdelivery:
driver = appendfile
file = /mail/spool/${domain}/${local_part}

The virtual domain password file looks something like:
username:Nt3AixExJlFZE:

Since all my mail accounts do not have actual system accounts,
I cant use operating system quotas. Is there an expansion expression
that I can use so I could have password file entries like:
username:Nt3AixExJlFZE:10M
that state that user username has a quota of 10megs?

Thanks in advance,

    -F