anonymous ftp + firewall

Hello, we have installed a SuSEfirewall with packet rules. The connections from inside to the internet are restricted to secure ports only. As a consequence the ftp port 21 is disabled. Now we want allow only anonymous ftp for downloading. Is this possible? We have tried to configure the squid proxy, but have found only restriction for soure/dest IP or time/user restrictions. ciao Jörg