I suppose you could, but it would be pointless. Like someone else already said, it would depend on if your software could recognize, or use 2 different interfaces at the same time.
It goes like this:
Instrument -> Microphone -> Interface -> Computer
The interface is where everything being recorded gets directed to the computer. This could be a soundcard, a firepod, or a USB interface.
Sometimes there is a preamp before the interface, after the microphone. But some interfaces have preamps built in, which can range in quality. External ones are not always needed but they are recommended.
A FirePod acts as an external soundcard. You can record into your computer, and play audio back through it. It allows you to hook up speakers (monitors), microphones, etc.
So since everything is already done with the FirePod (Firewire) there is no point in hooking up a USB interface. You could daisy chain a USB interface into the FirePod, if you have a USB/FireWire converter to hook on the cable, but the end result would still be firewire so it would be pointless.