FTP change source ip address

Dear everybody, 

we are currently facing an issue with FTP source ip address. I know that for some services (syslog, ...) it's possible to change source ip address, but i would like to do the same for FTP. Which seems t be really difficult (or maybe impossible?). 

In accord with attached image, we would like to open a FTP connection from fortigate 300D to send a backup to FTP server  behind a 1000D.

Ipsec VPN allows 192.168.0.0/24 and 192.168.10.0/24 networks.

Problem is, when FG300D try to connect to FTP, it use 192.168.1.99 ip address as source. I would like to be able to set 192.168.10.1 as FTP source ip address to be sure that it will be routed through IP-SEC vpn with a reachable ip address. 

All needed firewall policies are already implemented. No NAT is currently used on thesse 2 firewalls for this type of communications.

If you need more information, feel free to send a reply and i'll try to respond in short times.

Hope that someone have THE solution. 

Kind regards, 

Denis