Knitters are historically afraid of zippers, but they are very easy to sew in, and for this particular sweater, the zipper makes dressing easy as pie.
Zippers should always be sewn in by hand. The zipper should be the exact same length as the opening, to avoid stretching or puckering.Place the zipper in a stitch or tow so the zipper teeth will not be exposed. Pin the zipper with the right side showing, then baste it in with a simple running stitch. (Using a contrasting color thread will help.) Remove the pins and turn to the back side. Sew zipper edge with a backstitch. Repeat with the other side of the zipper, and you’re done. Be brave, and try it!