Hi,
You can create a content filter (incoming or outgoing or both) and specify action as bcc.
Do note that bcc action requires an email address where the email should be copied to. And it can be sent to an alternate host from the same action.
If the intention is redirect the original emails to the Fortimail appliance and then deliver it to the recipient you would need to update the SMTP routes to point to that appliance.
Regards,
Libin Varghese