Crowley's granite says that undermounting composite to granite can be done in a few ways. This is the most common method according to the well-known granite provider: "Stainless Steel and Composite sink manufacturers provide a system for attaching directly to the underside of the G/Q countertop. The system includes 6-8 bolts, clips, and wing nuts…"
"The bolts are inserted into grooves that the fabricator drills or cuts into the back of the counter, spaced evenly around the flange of the sink. The clip is then attached to the flange and tightened to secure the sink to the countertop." It sounds more complicated than it is, but if you feel that you need professional help, go ahead and hire an expert.