We have a kennel of working sleddogs plus one golden retriever and a GSP. The sleddogs can occasionally get too thin through hard work and we very gradually will up food consumption, but more often than not simply suppliment with a hard-boiled egg (shell and all). The dogs love them, eggs are very digestable, and its extremely easy to toss a couple in your coveralls during dog chores.
I do agree with the comment that 98% of the dogs I see are overweight. However, sometimes (mostly in the winter), too skinny can be problematic. Depending upon the quality of the dog food, sometimes just feeding more kibble can mean picking up more poop. Diamond green is good, not terribly expensive. We've fed it before. Right now we're feeding Kasco High-performance, but we suppliment with about 50% whole ground chicken. 50 pound blocks of frozen chicken are not very convenient for most folks, though....