yes, you can. but you're missing the point. a USB mic is a soundcard, but it's only ONE WAY. you'll still have to have some way to HEAR what you're recording or your backing tracks. there's no output on the USB mic so how are you going to hear what you're recording?

answer: your onboard soundcard OR your line 6. either way, that's a SECOND soundcard and you'll likely have SYNC problems because they have 2 different clocks and one of them has to be the master but the other is not designed to slave to that master.