You can certainly try it. If you have her under, say, a 150 HPS, then the lamp will provide enough heat to keep growth happening. However the greenhouse or at least the area around the plant needs to be kept at least at, say, 15C, and that's pushing your luck. I have done it myself without problems - but not with 100% success rate. I was only able to continue the line because I was revegging 3 moms at any one time. I did this for 3 years with a outdoor grow room, little better insulated than a greenhouse. Outside temps often went to 4C. You can put a thermostat in line with your intake fan so that if the outside temp goes too low the fan shuts down. Better a lack of CO2 for a while than a blast of freezing air!