One way I've found to extend the temperature range of my sleeping bag is to use a vapor barrier liner. But you don't want to be wearing much, if any, clothing inside the liner since then you tend to get moisture condensing in the clothes. I've found it to work well when the temperature gets below freezing in my 2 lb. (1 lb. down fill) sleeping bag that's rated to +20F (-7C) and have used it comfortably with the vapor barrier down to 0F (-18C). The liner only weighs a few ounces but makes a huge difference when the temperature is below freezing. An added benefit is that I lose less moisture during the night so when I wake up in the morning I don't have my usual thirst.