How about go to
http://www.askmehelpdesk.com/dogs/in...tml#post254171
It is highly unusual for a healthy dog not to be eating all it needs. There is a link in the sticky to how to evaluate your dog's body condition. If you have an adult dog near its ideal body condition, leave well enough alone. Both the sticky and the website linked to in it discuss the way overweight shortens a dogs life.
If your dog is still a growing puppy, again you don't want to fatten it up. Keeping a puppy lean and even slowing its growth with an early switch to adult chow allows more time to develop sturdy joints.
If you read though the sticky and its links and still think your dog needs to gain weight, post back with more details. A picture would help, see
http://www.askmehelpdesk.com/dogs/be...ghlight=Saluki