Even though I bought a size larger than I normally would, the sweatshirt jacket I made from the cheap department store sweatshirt turned out quite small and is shorter than I would like.  To add length before making patchwork, cut a piece of thin batting for back and front for the extra length you want (4-6" might be about right), butt to the sweatshirt pieces and zigzag together.  On the right side of the sweatshirt (that will be inside the finished jacket), sew a piece of coordinating fabric right sides together and flip over the batting.  Now measure and make patchwork!