Hi IShall,
Not everything is configurable in the GUI in FortiManager, but it can absolutely be managed centrally. Under System -> Admin -> Admin Settings, ensure Scripting is enabled.
Then go to Device tab, and you should see a Script section where you can create a CLI script.
Enter in the CLI commands you would enter in the prompt of a FortiGate, and choose the override target option.
From here you have two choices:
1. Run the script directly on the device CLI, which bypasses the FortiManager config DB, which may not be desireable
2. Run the script against the Device DB, which then preps it so the next time you push policy it will also push this config down.
Finally, if you are still on FortiManager 4.3 I would highly suggest you move to FortiManager (and FortiOS) 5.0.
Hope that helps
Cheers!
--
Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINET— High Performance Network Security