a quick google got me this http://forum.pfsense.org/index.php?topic=10834.0 when searching pfsense and ADM8515 usb NIC.
Can anybody tell me if this driver is in the current PFsense 2.0 builds?
I've got a HP T5540 (VIA C7 based terminal) that boots pfsense just fine of a USB stick (1GB Nano at the mo) but of course it only has the one onboard network interface. dug up a USB/NIC using the ADM8515 chipset, but it doesnt seem to work. pluging it in brings up a message on the console that reads "ugen3.3: <moschip semiconductor="">at usbus3".</moschip>
It doesn't look like that is supported under FreeBSD 8.1, which is the base code for pfSense 2.0.
ugen is what FreeBSD labels usb devices for which no specific driver exists.
One way around this would be to use a switch that supports VLANs. That way you can use VLAN interfaces in pfSense, as many as you need.
bummer. oh well, thanks for the reply.