You'll need to migrate the public folders separately. with the PST import tool.
Here is what we plan on doing and it works in our test environment.
1) from outlook connected to the exchange server, create a personal outlook data file (pst)
2) copy the public folders to that personal data file
3)create a zimbra user called public_folder and give it a password.
4) use the pst import tool and import the pst file into the public_folder account.
5) then set the sharing of the public_folder so it has the permissions you need. Can do this via command line or web client logged in as that user.
Rhel 5 64bit
zimbra network eval 6.0.5
Clients Win XP SP3/ firefox 3