It depends on the plant and your location. I over winter quite a lot on mine: pelargoniums, fuchsias, palms etc. I even over wintered a bunch of cannas in pots, they were fine!
If you don't have that many plants to look after, it may be possible just to section off part of the greenhouse for them. I find the main problem is damp, rather than cold.
I'm thinking of plants like streptocarpus and tender succulents. I've got away with potted half-hardy plants in the past, lemon verbena, some salvias etc. All cut right down and left pretty much dry. Based in Oxfordshire so not the coldest part of the country.
Tender succulents should be fine. I keep all of ours in our unheated greenhouse in Winter with no problems. You need to reduce watering to almost zero. It can be warm in a greenhouse on a sunny Winters day, so you will need to occasionally give them a tiny bit of water, to stop the soil completely drying out. I usually do this every 2-3 weeks or so.
With succulents it's often the damp that gets them in the winter rather than the cold. Ones with fuzzy leaves or crowded stems are especially prone to fungus problems. You can do a bit of research and find out where your plants are from and if they have to tolerate frost there. Some of my Mexican succulents should be heat loving plants but are actually from high elevations that get very cold for example.
If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
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There are various ways of protecting them, a few tips here
https://www.gardenersworld.com/plants/keeping-the-heat-in-your-greenhouse-in-winter/
If you don't have that many plants to look after, it may be possible just to section off part of the greenhouse for them. I find the main problem is damp, rather than cold.