You can add a simple, casual collar to a sweatshirt jacket.  First, before sewing on binding, staystitch the neck opening - stitch about 1/4" from the edge, taking care that you do not stretch the fabric.  Use a short stitch, such as used for patchwork.  Clip at 3/4" intervals up to the stitching, but not through it.  Measure the opening and cut two 3 1/2" wide rectangles that length.  Baste batting to one piece, or use fusible fleece or interfacing (your choice).  Sandwich the neckline between the two collar pieces, right sides of collar toward jacket with the plain piece to the wrong side of the jacket.  Sew in 3/8" seam and press away from jacket.  Quilt if desired and bind entire jacket raw edges, including collar.