You thread a nice long piece of yarn, some cotton yarn is nice, but most any smooth yarn will do, though a blunt needle you use for seaming. Then use the needle to run the yarn through right where the circular needle is. You can hold the circular in your left hand an run the sewing needle into a few stitches at a time purl wise. Make sure you have a plenty long piece so that when you try the sweater on your stitches don't fall off the ends.
Then when you are ready to knit again, feed the needle back in where the yarn is. You may like to use a smaller circular to feed though and then begin knitting off with your needle that gives gauge.