Mite, you know enough to keep a healthy dog and it is all good advice.
Diamond makes a good dog food... it has a variety of foods that you could choose from. Hi energy and extreme athelete have upwards of 30% protein and 20% fat. However, the chicken and rice has a main ingredient of chicken and has a 26/16% on protein/fat. Also, what I do to get my dogs wight up prior to season is add a premium canned dog food to her dry food. I usually mix 12 ounces of canned per serving of dry. One thing tho is to not get your pup used to this or it will only want dry/can mix.
excersize is still very important as you want your pup to not only gain fat reservs but also lean muscle ( it weighs more than fat) My dog runs for about 15 minutes and swims another 15 minutes every few days prior to her dinner by the time we get back to the house she is calm enough to eat.
Something I have heard and seen is to allow your dog to have a free diet, this means to have food available for it 24/7 as a dog will know when it has had enough to eat. However, my buddies lab is obese due to this. post season his dog went from 94 lbs to 120lbs.