Ian
Did you read my reply carefully.
It does give you an alternative way to achieve what you are after by setting up additional ibays which are password protected. All you would have to do is move the existing files from \server\primary\files to \server\newibay\files and tell users the new URL ie
ftp://newibay@ftp.mydomain.com. You could put a link to it on your existing web page.
If you want users to use their own accounts & passwords then you need to get them to VPN to the server.
Regards
Ray Mitchell