This is just my opinion. I think that the only athletes that should be gaining weight are football players, weight lifters and sumo wrestlers. Bike riders and the average person should be as skinny as possible. Again this is just my opinion.

These foods wont meet all of your requirements but should be calorie dense.
Cheese and macaroone
Ice cream
corn bread and butter and sugar
cakes
fruit pies
candy
pork / bacon / all the lard that you can eat.

The above should add weight and give you a coronary in short order or at least start you down the path to type 2 diabetes.

I really think that you should be happy with your body and realize that there are many people trying to get where you are. Just take a look in the clydesdale forum.

I don't know your age but most peoples metabolism slows down around 40 years of age and the weight gain comes more easily.