How to gain weight but stay thin?

i have anorexia and im 16, i weigh 7 stone 3 pounds (101 lbs) and i am 5 foot 7. i look like i weigh more than i do but i also understand that may just be the illness causing me to think this. but now i have gotten to a stage where finally i am more or less happy with the way my body looks. i am a dancer though and have been stopped from dancing! i have to get my weight to 8 stone before i am allowed to dance again. i do want to do this and am doing very well so far eating 3 meals every day which is a huge breakthrough as i used to eat just once. how can i get my weight to 8 stone veryyyyyyyyy fast but not look different at all ? thankyou very much x