Singlespeed Hub - Rear Axle Swap
http://surlybikes.com/parts/hubs_v1 says "This is a common size so it’s easy to find replacements should the need arise or to swap axles if, for instance, you have a QR axle and want to go solid", so it should be easy enough.
Based on the photos, you might want some cone wrenches even though with cartridge bearings there aren't actually cones as such. As the Surly page says, you still need to adjust the load on the bearings when you do it up.
http://sheldonbrown.com/tooltips/conewrenches.html