Assuming Fax works on the MFP:
1. Download the PCL driver for the copier (not the fiery)
2. Install it to the IP of the Fiery, but set it to RAW, and change the port from 9100 to 9200 (Nothing else selected)
The changed port number will go to the Fiery, and due to port 9200, will forward it straight to the copier and the Fiery software will not display it.
Then you can Lan fax from here;
If you grab the PS driver there is very little chance of success, Postscript is an add-on to the printer and it would be pointless to buy when you have already added the Fiery. This driver will not have any Fiery features and if the port number is not changed the jobs will print corrupted and with none of the finishing or layouts you choose.
Drivers are here.
Pick the PCL driver, not PS, and not the global, only the PCL6 driver