Try sticking a quill stem in your fork first, and if it doesn't fit, you'll have to have the fork reamed out. Most local bike shops won't do this, and I had to take a 1" threadless fork to a local bikebuilder to have them ream it out (they also built the fork). I'm not sure if Somas are typically thicker than usual or not due to them being threadless.

If you do have the local bike shop thread your fork, make sure you bring them the headset and the frame so they can cut the fork to the same stack height as your headset. Your best bet would be finding a local bikebuilder and having them do this for you, 9 times out of 10 they'll be more precise.