Enzo,
Good job getting your doctor in on your desire to lose or convert 10 lbs to muscle. Some stellar athletes who also have CF have given you motivation and some starting pointers. The whole point seems to get you off dead center. Don't get in/on any motorized vehicle for a few months. This may amaze you but walking farther than to the garage can do wonders.
Have you purchased your weight set, investigated and purchased a book or magazine on body building, checked out whey and soy Muscle Milk, started a daily workout? What have you actually done to convert this body mass? Thankfully other contributors have poked you in the backside in hopes you leap from the nest. Fly or quit chirping about it.
Here's some fun. On a Friday evening after good hydration and a Power Bar, have your friends keep you ankles down and do sit ups until you can't do another one. Drink some Gatorade, eat a banana and repeat the task in an hour. If you can get out of bed the next day, do some more sit ups. Rest a couple days and bingo, instant six pack. At your age, weight is a premium but until your late 20's six packs in a week are possible. They won't be bulging masses of masses, but you will define and tighten the abdominal muscles so any bloating is kept behind a wall of muscles.
If you hope this is going to up your status among your peers and make girls swoon, you'll need a different tack. Girls may hope for broad shouldered muscular guys, but any broad shouldered, muscular guy obsessed by their own body image would turn all but the most vain of women, off. Body builders recognize that you are stalled. It happens when you fear that a whole bunch of work will result in nothing. If you belive that, give up.
LL