Multiple user with vpopmail

[Sebastien VIEILLARD]

I'am an isp and i want to use vpopmail for any site installed on server.

The problem is that any site have one specific unix user. and of course, any user have 
a limited quota.

But vpopmail create mailbox for only one unix user (vpopmail user by default).
Is anybody know, what i can do to have each mailbox ~vpopmail/domains/domainename, 
under specific unix user.

thx