I added a buttonhole to a quilted jacket as an afterthought. Here's how:  Cut a bias piece of coordinating fabric  about 2" wide and 2" longer than the finished hole you want.  Mark a line on the wrong side the length of the button + 1/4" and place it right sides together over the place you want the buttonhole.  Stitch a 1/4" wide box around the line with tiny stitches.  Use sharp scissors to cut neatly between the lines of stitching, then turn the bias piece to the wrong side; press.  Stitch in the ditch around the hole, then hand finish on the wrong side.