If your winter camping gear is insufficiently warm for the weather, your nights will be miserable. You shiver, you do isometrics inside your bag, you toss and turn, and it's hard to fall asleep. The night seems to last forever, and you aren't rested when the sun finally comes up.
What you want is a combination of gear which is sufficiently warm so if you bundled it all on, you would be overheated, and you need to leave off some clothing and leave your sleeping bag's hood loose and zipper partly unzipped. If it gets colder at night, you can wake up partly and zip up tighter.
I think that keeping warm at night while winter camping involves four things: (1) a four season tent, (2) a thick winter sleeping bag, (3) an insulating sleeping pad, and (4) clothing which you wear while sleeping.
You want a four season tent which can be sealed up completely, except for ventilation holes. This kind of tent can add a lot of warmth. I like tents which can be vented near their peak. A tent with panels of mosquito netting which can't be sealed will not be very warm.
You need a thick sleeping bag with a hood which seals comfortably around your face, and a collar which keeps cold air from zipping down into the bag. If you want to sleep well, don't skimp on the bag! If your bag is inadequate for the weather, you can add insulation by laying blankets or a quilt over your bag, but this is not as satisfactory as having a really warm bag.
Your sleeping bag won't provide as much insulation underneath you as it does on top of you, because its insulation will compress. You need a sleeping pad or pads which will insulate you from the snow underneath you. A thick pad will help you keep warm at night.
If your sleeping bag by itself doesn't provide enough insulation to keep you warm at night, add clothing. A thick balaclava and a couple pairs of socks will be very helpful. Put on a couple layers of warm clothing on your upper body and lower body. There may be a limit to the number of layers you can wear on your upper body, as eventually the layers will bunch up in your armpits and impede the circulation to your arms. If this happens, lay a winter jacket over your body inside your sleeping bag, which will add a lot of insulation. If you lay the jacket on top of your sleeping bag, it'll probably slide off as you turn over at night. If it's inside your bag, it'll stay put, and you can easily shift it around as you turn over.
That's how I keep warm while winter camping. I think it would be hard to develop a satisfactory winter camping setup based on a hammock. Open_side_up's suggestion that you try out your winter camping gear in your back yard is a good one. I remember a weekend when I had planned a trip, but a blizzard blew in and the authorities closed the highways. I decided to camp out in back of my house. It was -30 degrees F, and the wind was howling. It was a struggle to put up my tent, partly because of the wind, but mostly because the tent's nylon shrunk from the cold, and I had a hard time installing the poles. But once I got the tent set up, I spent a comfortable night outside.
Here are some pictures from a four-day 116-mile kayak trip on the Middle Fork of the Salmon River in Idaho, launching on Marsh Creek, where we kayaked through snow for the first day and a half. Due to the limited space in my kayak, I brought a three season sleeping bag, and augmented it with the clothing I wore under my dry suit during the day. I was warm and comfortable at night.



