First of all: If you attempt winter camping make sure you have a real winter sleeping bag, with a comfort rating to fit the lowest night temperatures you are expecting, plus a safety margin.
And yet....maybe your sleeping bag is not quite warm enough? You'd like to have a few extra degrees of warmth, without having to buy a new sleeping bag?
Use a second sleeping bag, a sleeping bag liner or blanket to improve the insulation of your sleep system.
Mind you, two thin summer sleeping bags don't make one warm winter sleeping bag. If you have a three to four season sleeping bag, which didn't feel quite warm enough last time, and you have another sleeping bag then you can stuff one into the other and improve the insulation by quite a bit. A fleece sleeping bag liner can add a few degrees of extra warmth too. The same with a blanket. Wool is better than fleece. Put the blanket inside the sleeping bag. Synthetic sleeping bag covers are so smooth, everything you put on top will slide off when you move and roll around. Best practice is: 1) open your sleeping bag and spread the blanket on top, 2) lie down on one half, then fold the other half over your body, 3) close the sleeping bag. Thus you get a bit of extra insulation below you and on top of you. It may take a little bit of time to get used how to move without getting entangled in the blanket each time you turn and toss, but it usually works fine for me.
I also used my very lightweight downquilt to add extra warmth to a sleeping bag. I stuffed it inside the sleeping bag, thus giving it a considerable extra loft and insulation.
I also have an extrem winter set up. I use my thin summer sleeping bag as a protective cover over my expensive winter sleeping bag. My down winter bag was expensive. The synthetic summer bag, on the other hand, is a cheap one. I rather have the cheap summer bag take the abuse, dirt and wet. Here is how I use it: 1) I close the lower 1/3 to 1/2 of the summer bags zipper, 2) pull it over the foot end of the down bag, then spread the upper half like a quilt over the top, with the sides loosly falling down over the winter bag. The outer sleeping bag is held in place, without constricting the loft of the inner down bag. It protects the winter bag from moisture, dirt and flying sparks from the campfire and add some extra warmth.
Winter camping in an open lean-to shelter.
Using multiple layer of sleeping bags and bivy bag to increase the insulation and warmth of the sleep system.