If the interfaces weren't added during the installation process, it's likely that the drivers weren't even loaded. Your best bet is likely to just reload from CDROM when you have configured your hardware. If you want to add a NIC later, just be sure to pick one identical to one already installed.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005