First you should contemplate what the track record is of that type greenhouse in your exposure of site and your region. Serious solid commercial-farm-quality hoophouses with the proper covering generally do fine, but cheaper things, or re-covered in inferior plastic, go "splat" with winter wind and snow in many areas of the country. And if this is just a little garage-type frame, all the moreso.
If you ARE comfortable that the thing will not go splat (or whoooosh-hey-look-its-in-the-neighbors-cowfield) over the winter, the other thing you need to consider is ventilation. It is a bad idea to try to use a CLOSED greenhouse-style structure for chickens over the winter. You would want to leave a substantial amount of space open for ventilation. Otherwise you get really bad frostbite-producing humidity problems. The best or easiest way to accomplish this will depend on exactly what structure you're working with, but at a MINIMUM I would say convert the people door to an all-mesh door, and in even a modestly-decent sized greenhouse that will still be insufficient. If you can make one whole end open (mesh), that would be really really good; but then you have to worry about wind issues more.
Good luck, have fun,
Pat