The only way I can think of acheiving this is if you somehow forced the FTP traffic through Squid. This is no problem if you have setup your browsers to point at the proxy server, then they'll automatically go to squid for FTP. But if you've got it set to transparent then you'll need to find a way to redirect the ftp trafic into squid. I can't think of one off the top of my head, but maybe someone else can.
The only way I can think of acheiving this is if you somehow forced the FTP traffic through Squid. This is no problem if you have setup your browsers to point at the proxy server, then they'll automatically go to squid for FTP. But if you've got it set to transparent then you'll need to find a way to redirect the ftp trafic into squid. I can't think of one off the top of my head, but maybe someone else can.