Passing client IP's for FTP

Imagine you have more and less this topology

(F5)-(Router)-(FTP)

One approach:

1. One static route in the F5 to reach de FTP server
2. Configure FTP gateway to use your router IP
3. Configure a PBR (policy base routing) in the router to send all traffic which comes from FTP server to the F5

With this idea you should have end-to-end communication between your FTP and clients.