If you have the XLR adapter and are plugging it in with that then I believe you can set one channel to run into both audio channels. If you're using the on-camera 1/8" jack then there's a couple things you can do:
One: If your mic is an XLR mic and you're using an adapter to go from the mic to the camera than you need to cast around for an XLR to balanced 1/8" adapter. They're a little bit tricky to come by in standard audio stores, but if you look around online you should be able to find one. (You will record the audio to both channels then, but the mix won't be truly stereo)
Two: If the mic has the 1/8" unbalanced jack as part of the assembly then you MIGHT be able to cobble together a mess of adapters that will allow you to run the unbalanced signal into a balanced jack that you can plug into the camera. The issue with this is that when you split the signal through adapters like that you can experience signal loss.
Three: Do what everybody else w/ an unbalanced Mic does. Plug it in and offline just your 1st channel of audio when you dump your video. Then take that Audio Left channel and balance it to Centered. No, you don't get a true stereo mix then, but depending on what you're doing that shouldn't matter. If getting a stereo mix is critical then you need to buy a new microphone and get one that specifically says it's stereo.
Hope this helps.
