The short answer is probably not easily. Technically it would be up to the software that you use. But every DAW software that I can recall using only allows 1 audio input device at a time. That is, in the settings for whatever recording software you use, you'll be asked to pick between either the audio interface or the USB mic.
If you happen to use Mac OS X, there may be a way through the use of an aggregate input device and/or Soundflower. So you'd then have a new "virtual" input device with 2 channels, one for each of your real inputs.
But honestly, I'd probably just pick one or the other. Check what kind of latency you get with your USB mic. Some of them induce quite a bit of latency as I understand it. If that's the case then take the opportunity to upgrade to an interface that gets better latency and probably has a better preamp. Or if the USB mic is working for you, don't worry about it, just use what works.