Adding extra nutrition w/o adding bulk

Could you maybe add some canned food or cooked chicken & brown rice?  Adding extra protein (and good carbohydrates) is a good way to add bulk for high-energy dogs.

Also, what food are you feeding? You may need to switch to a food that has more kcal per cup so that you can feed less or the same amount and add more calories.

I agree with checking out some foods that might have a higher energy level per "serving."  Compare Kcals on the bags.

For people who are really struggling with this issue - ie, a dog that is a poor doer and refuses to eat an increased allowance - there's something called "Calf Manna" that a lot of working dog people use to perk up their working dogs' appetites.  You get it at feed stores or Tractor Supply Company.  Just a teaspoon packs a huge amount of energy and nutrients, and it is baited with anise which most dogs find irresistable (black licorice flavor).  I don't use it because it would be too easy for my allergic dog to do real injury to himself if he got into it - it's that attractive that I'd be afraid he'd tear something apart to get to it.[8|]

You can also order Calf Manna online, by the way, if you live in a city nowhere near a feed store (you'd be surprised, however).

You might consider a working dog formula. Eagle Pack has Power and Nutro has High Energy. And there are brands marketed for sled dogs, such as momentum. You can also take a hint from mushers and add meat, cooked or raw, to the diet to up the protein/fat ratio. Keep in mind, though, that professional mushers usually have team's diet supervised by a vet.
 

We're currently feeding Eagle Pack, but my pet store doesn't carry anything with a higher content than we're feeding.  I would prefer not to order online, as I can't seem to plan far enough ahead to have some on hand when I need it.  [8D]  And his appetite is good.  There really isn't much he won't eat. 

How much meat can I safley add?  What kinds of raw/frozen meats are safe?

you might want to stop with the soy powder-- dogs can't really digest soy-- it mostly just produces gas in dogs. Some Dane people believe soy should never be fed to dogs because it seems to be linked to bloating.
Have you thought about getting some Zuke's power bones or  Evo biscuits or something similar and just feeding a few throughout the day?

Ahh, I didn't even think of "treats".  That's a good idea. 

Some people that I train with give their dogs Nupro Electrolytes.