I had a pair of purple low-rise sweat pants that I had never worn as they started about 6 inches below my belly button (bikini height). My preferred height is a more comfortable one at my natural waistline.
I took a burgundy, felted wool sweater, turned it inside out, cut it in a curve, underneath the sleeves, that matched the curve and width at the top line of the pants. So I had a tube that was about 12 inches tall, I planned on folding it over like yoga pants. The wide part joined the hip, and the narrow ribbed part faced upwards.
I pulled the drawstring out, left the soft elastic in. I turned the pants inside out. I used a couple pieces of cardboard to keep the front & back separated and pinned the inside out sweater--about 6 pins on each side was plenty. (In retrospect, I shouldn't have turned it inside out so the seams would not show when folded over. No biggie though, since most likely a sweater will be covering the tummy.)
I hand sewed the cut edge of the sweater to the bottom seam of the pants' existing waistband, removing one pin at a time as I inched forward and around. I thought of doing a second cycle around the top edge of the pants existing waistband, but the union seemed sturdy enough. I tried them on and they felt so much better in comfort and modesty.
I took my scissors to the sweater again and cut the sleeves off above the seam, and voila! a pair of cozy armwarmers. Just a dickie-sized remnant left--perhaps future warmers with a soft cotton lining?