My central office and remote offices are connected to each other over site to site ipsec vpn. We use Fortigate firewalls and everything is working fine. On the other hand, only central office is also connected to another company's network over ipsec vpn as well. In this situation, everything is also fine and employees at the central office is able to reach the other company's resources without problem. Now i want the employees working on our remote office can reach the other company's network over central office without creating new vpn tunnels.

http://imgur.com/ozrXfGv

How can i do that? Thanks for your answers in advance.