You could use something like this to keep under your coat in close proximity to the camer to keep the ambient temperature above freezing point
Moving cameras in and out of warm humid places in cold weather invites condensation. It's usually better to keep the camera at the outside temperature, and seal it against condensation when bringing it indoors.
Benjiboy is right :
Condensation happens if warm humidity is in contact with cold hardware. On the opposite, when the hardware is warm and the air cold, there is no risc of condensation.
When I go skiing, I always keep my camera under my skidress, thus close to my body temperature, and I pull it out only to take photos, so that it has no time to cool too much. This way, I never experienced any condensation problem, never any mechanical issue due to freezing of the lubricants and never any electronic malfunction due to too cold batteries.
But when you go back to your well heated house, you may get some condensation, because the air there is warmer than your camera. In this case, do nothing, just let the camera get the room temperature and the condensation will evaporate.