Upgrade AV database on an isolated FortiGate

Hello,

I have a FortiGate which is isolated from Internet (and Fortiguard). I need to update the AV database, but it seems this capability is missing in 6.0. The FortiGate is licensed for AV and up till now we did the AV updates manually. Can I do that in CLI? I don't mean exec update-av, because this command will update the AV db from FortiGuard. I mean a CLI that will offer me the option to specify the av file manually.

Thanks