[Dovecot] How to serve two "inboxes" via imap

[Roderick Johnstone]

Hi

In our setup we have mail delivered to /var/spool/mail/<username>.

Many users like thunderbird as their MUA and we typically have 
thunderbird set up to read mail using a movemail account with the local 
mail directory set to ~/mail so that the mail can be read easily by pine 
as well.

So, when a user clicks the "Get Mail" button in Thunderbird, mail is 
moved from /var/spool/mail/<username> to ~/mail/Inbox.

Although I can serve other folders from ~/mail my imap clients don't see 
~/mail/Inbox, they see just /var/spool/mail/<username> as INBOX.

Can anyone advise please how to make the ~/mail/Inbox available as well 
via imap using dovecot (currently using 1.0-alpha5). We currently use
default_mail_env = mbox:~/mail/:INBOX=/var/spool/mail/%u

Many thanks

Roderick Johnstone