Forti OS Update in Batches

Is there a method to update a batch of FortiWiFis using a TFTP server?
I know i can use a TFTP server to recover bricked FortiWiFis, but is there a method or tool that does the same but for a batch of FortiWiFis?
In my case I want to update as many as possible and then put them aside as stock for future device deployments. And so I think adding them to FortiManager is. not ideal. 
 

Basically I intend to downgrade from 7.0 as that is what they now come out of the box to 6.4.14.