Reverse Invoke and FTP

We are using B2B 4.02 and TN 4.5. The B2B Server is setup in a Reverse Invoke configuration. But, now I realize that some of our customers want to FTP the documents. Has anyone solved this issue and guide me in how to route the documents FTPd to the Reverse Invoke Server (on a regular FTP port as I can’t setup a proxy port for FTP) to the internal server as the customers can’t directly access the Internal Server.
I appreciate any suggestions/comments.

We use a reverse invoke set-up also.