There's several threads in the forum about fetchmail,
this is the one that describes how to set it up. Being a linux novice, I use webmin to manage my server and it has a nice module for setting up fetchmail. If you don't know webmin you can find it at
www.webmin.com
If you have the subdomain set-up in zimbra with the same account names (or aliases) then fetchmail should drop the mail in the correct mailboxes.