VirtualBox 3 will allow you to have up to 8 virtual interfaces per VM. 4 of those you can create and control via the GUI, the other 4 you will need to use the command line for. No fear, Virtualbox makes this a very simple operation.
Here I’m adding virtual interface number 5 to a VM named pfsense. I’m setting that interface to be bridged to eth5 and making sure that the virtual cable is plugged in.
VBoxManage modifyvm pfsense –nic5 bridged
VBoxManage modifyvm pfsense –bridgeadapter5 eth5
VBoxManage modifyvm pfsense –cableconnected5 on
It’s that simple. You can use all of the connection types (nat, etc) and even change the type of nic hardware. It’s all fully documented on page 110 of the user manual.