I disagree, it is possible. You would have to manually set the networking settings in FeeNAS for the internet facing port to a null configuration. You could even set it static for addresses outside of your network and the public network and use a subnetmask of 255.255.255.255 This would work as long as FreeNAS uses bridged network virtual adapter configurations. Alternatively you could also see if the USB passthrough in Virtual box is available in the FreeNAS version of VirtualBox. Then just a pass the internet to the vm using a usb dongle. The issue here is that you don't want FreeNAS using the usb network before VirtualBox loads. You could intentionally break the driver in FeeNAS. The main issue is your cable modem can't get two MAC addresses.
Possibly the best way is with VLAN tagging. You would connect the modem to an enterprise switch and then configure it to tag that physical port with a VLAN tag to the port connected to the FreeNAS box. So if the VM was not running, then the VTAG should be dropped.
And then there is the way I'm doing something similar, virtualizing FreeNAS, pfSense, Ubuntu, and possible OS X using ESXi. But that has become expensive and complicated. And I'm pretty sure the SAS controller is using as much power as a low power computer would by it's self. LSI should have put a bigger heatsink on it.
For a home a low end ATOM based motherboard that uses a 19V laptop power adapter is the best way to go. There are some with dual networking just for that reason. A used netbook that can boot pfSense would be even slicker. Have to use a ASIX based usb adapter. Fora home you don't need much for pfSense. The requirements they list are excessive. I have it running great with just 1gig of ram.
The next version of FreeNAS is supposed to bring hypervisor support and then what you want to do will be even easier.
I guess it really depends on how much work you want to put in to it.