I need to attach a 4" circular pan circuit box to a steel stud in order to hang a bathroom vanity light fixture. The fixture weights less than 10 lbs, but assume some future homeowner will want to install a bigger fixture -- but maximum of 20 lbs.
If the steel stud is of sufficient thickness/strength you could fasten it with self-drilling Tek screws. I would personally be more concerned with the weight bearing capability of the steel stud (assuming it is the lightweight steel studs commonly used in commercial build-outs in my area).
Most ceiling fixtures (you use the term stud rather than joist so I am assuming wall mount) that are expected to hold any weight should be attached to a cross-brace which is attached to two joists.
Consider using a long pilot drill and drill through both the front and back sides of the steel stud, then use a long self-tapping screw to penetrate both sides of the stud.