surgery would be the only way to put on 12lbs of muscle in 6 weeks. not even steroids could help that much. its just not physiologically possible.
keep this in mind. pro bodybuilders may put only 10lbs of muscle in a year. these people are on all kinds of supps and vitamin S and they live to get bigger. something most of us cannot do.
now you can put on that much bodyweight in that time frame. just not in muscle.