You can propagate magnolia by air layering. It can be a long process though. You can also take cuttings or grow from seed. From this site:
Propagation: Magnolias can be grown from seeds which often vary in crown structure, size, vigor, and flower and foliage characteristics. Freshly cleaned seeds may be sown right away, but germination may be improved if seeds are stratified at 40 ...°F for 2 to 4 months. Cuttings from soft to semi-hardwood from juvenile plants should be used to propagate magnolia trees. Cuttings should be treated with a rooting hormone and placed in a moist environment. Treated this way, they usually root within 6 to 12 weeks.