I remembered VirtualBox on Linux has "USB passthrough": a VM can take control of a USB device. Hence you could (if supported in VirtualBox on Windows) add a USB NIC, assign it to your pfSense VM and connect that NIC to your "rest of network" so that traffic from that part of the network has to go through the pfSense VM BEFORE it can get to "server1".
pfSense is not real good with dynamically appearing interfaces so you will probably need to reboot the pfSense VM a couple of times to ensure the USB NIC is correctly seen on pfSense startup.
There are a number of USB NICs that are supported by FreeBSD/pfSense which say they are USB 2 compatible but don't say they are not capable of 480Mbps operation (that is, they talk to the host at only 12Mbps or lower). Depending on the speed of your WAN link you might need to choose the USB NIC carefully.
USB NICs don't have a great reputation in the pfSense community. I suspect at least a part of that is from people not considering all the details. I used a USB NIC for a while and eventually ditched it because it wasn't reliably seen on startup which meant I sometimes needed to be around to fix up the situation if pfSense restarted. I could probably have tweaked pfSense to get around that but I had a VLAN capable switch which I was able to use to effectively get extra ports removing the need for the USB NIC.