Maybe you should tell what do you mean by "secure ftp site", because I think this expression could be very confusing.
Do you mean any sort of user interface which allow users to upload files, but with a dedicated client software (not a web browser) ? Or precisely using the FTP protocol (File Transfer Protocol) ? Or one "Secure FTP" protocol, as described in http://en.wikipedia.org/wiki/Secure_FTP#Secure_FTP ? (SFTP is the most used).
If you mean using a specialized protocol (FTP, SFTP...), it can't be only a Moodle feature, as you need a server for this protocol. If it is possible, it would certainly be a kind of Moodle Repository, see http://docs.moodle.org/23/en/Repositories
But are you sure that the standard "Upload a file" repository (via the browser) doesn't fit ? Because if you choose a solution based on an external server, you will have to maintain a second reference for user accounts. Maybe possible for a few users, but it seems not very scalable.