Yes, use /etc/mail/virtusertable to map virtual users to real mailboxes on the system. This link
has an example. I've used virtusertable combined with aliases to set up truly virtual users, e.g.:
Then you can instruct your POP3/IMAP server to use /var/spool/vmail/my.virtual.tld/foo/mail/inbox as the inbox for user firstname.lastname@example.org
using whatever configuration method is appropriate for your particular server software. This may be more complex than what you're looking for if you just need the virtusertable to associate an address with a "real" system user (i.e. one with an entry in /etc/passwd).