You would have to know the temperatures you are dealing with, what are the night time lows in the cold frame? You also have to consider that only the cool to cold growing species would do well and then most of those would still like a day time climb in temperature. Perhaps you could start some cuttings of some of your plants and try them in the cold frame. I would not suggest attempting if you think the temperature would consistently be below 10 degrees C. Also care during times of prolonged cold weather would be difficult, watering under these conditions could very easily cause problems.

Mike

the temperature sometimes get to -2 for 1 day and then back up to 10-15

Well without controlling the temperature you will not be able to keep Hoyas in those conditions. Hoya burmanica can take frost but other than that the more common Hoyas you find in collections would turn into mush at those temperatures.
If you use a heater to keep the night time lows from dropping too far then you could very well keep your cool growers out there. A fan for air circulation would also be necessary.
I think that a trial run with some cuttings would be a good little experiment to run because it you are successful then you will have that much more space to grow your plants. I know it's hard for you to get Hoyas but if you can why not search out as many cool growers as you can so that your collection is best suited to your growing conditions.

Mike

what if theres compost at the bottom?to keep the heat from the day in

Do you mean active compost to generate heat? I am not quite sure what you mean. You can use tricks for heat storage such as a large dark container of water that will heat up during the day and then release heat at night but you can not use those methods alone. Backup heat is necessary unless you want to risk loosing the plants.
Is this a small greenhouse or a cold frame that you use to start vegetable seedlings in the spring?
You can get nice waterproof propagation heat mats for starting seeds and those might be useful beneath pots to help keep the temperature from falling too low.
If a structure is not double walled you will have a very hard time keeping heat inside due to lack of insulating air space.

Mike

Yeah thats what i meant about the heat and yes it will be like a mini glass house. I am in zone 9 if i was in America

I would suggest visiting the greenhouse forum and asking there. I am not familiar with greenhouse growing or growing in that climate zone. You might be able to get away with minimal heating on just the coldest nights. There are lots of tricks to maximize the capture of heat during the day as well. I don't think the compost idea is reliable so it's not worth the risk of loosing your plants.

Mike