It's possible to setup forwarding specific mailboxes to Fortifi in any Domain Controller. For Google Control Panel you can follow these instructions:
Log in to https://admin.google.com/ac/apps/gmail/routing?hl=en
1. Create a new routing setting, and select this to affect Inbound Messages, with an Evelopes filter to only affect specific envelope recipients - set to Pattern Match: mailbox(\+.*)?@domain
2. In section three of the routing rule set the Envelope Recipient to Change envelope recipient -> replace recipient | mailbox@organisation.fortifi.cloud
3. Finally select the Account Types to affect with this Routing Rule: set this to Users, Groups and Unrecognized / Catch-all
Repeat step 2 for each mailbox you wish to use.