I haven't tried this as we run ePO, but I have had a look and it should be possible to update from a local FTP.
First off you will need to open the VirusScan console and select Tools - Edit AutoUpdate Repository List.
Click "Add" and then fill out the options, you can select to retrive files from a HTTP repository, a FTP repository, a UNC path or a local path.
Once you have finished it, click "OK" and then move your new repository up the list to first and you should be right.
You can download your DAT files from ftp://ftp.nai.com/commonupdater/