Crystal
just make sure you dont show the seams, its tacky.

kay
Seems like a lot of work for very little gain. Remove the neckline ribbing of the sweatshirt. Mark the intended seam allowance at the neckline (I'd suggest 3/8" or 10 mm) and then measure the length of that seam. Draft a hood pattern to fit that neckline. Cut the pattern out of matching or contrasting sweatshirt fleece. Construct and sew the hood to the sweatshirt, finishing all seams. Be very careful not to stretch the neckline as you sew, or it will look bunchy and irregular during wear. Or go to Walmart and buy a hooded sweatshirt for $15.