1. Smoothwall probably does not have a correct driver for your USB network device. Especially true if it's a wireless device.
2. No, it's not possible to have two interfaces on a single NIC.
You can add your own drivers to an existing Smoothwall box by compiling the driver and adding the utilities required to bring the wireless interfaces up or installing a homebrew mod from the smoothwall community forum.
I would check the smoothwall mod section over at their forums here
. Also, when planning a purchase of smoothwall compatible hardware, you can look for your hardware in the compatibility list here