Stick with puppy food, it has more nutrients and vitamins and such. Make sure it's a high quality food also, from a pet store like PetsMart or a specialty store instead of Wal-Mart. You can also give them a little bit of whole milk mixed into their regular food (but not very much!). However, it is not good for a dog to be fat at all. It is much more unhealthy than an overweight human being and can lower their life expectancy by a few years.
Depending what breed it is, as some breeds do not bulk up, as genetically they have been bred to be streamlined. A thin dog is much healthier than a fat one. You do not say however, how underweight it is and if it is eating enough.
If the dog is a picky eater and has little appreciate due to illness or his disposition, you could slowly and gradually get him or her, to have a little chopped up liver or beat up an organic egg and add it to the food. A table spoon of cod liver oil is a good tonic once a week if you can get him to take it. Also add a spoon of olive or sunflower oil to the food once a week to keep fatty acid levels healthy and keep the coat shinny.
Do not be tempted to suddenly give a lot of rich and fatty food as this will cause upset tummy and vomiting. Dogs do not tolerate changes in diet very well, and what may seem boring to you, will be quite good to his hypersensitive nose. Consult your vet if there is no change.
Some Siberians are just thin, but sometimes people think they look too skinny when they are at a healthy weight because they are used to seeing Siberians with a full double coat for winter. When they don't need tons of hair to keep warm in the extreme temperatures, they may shed the extra fur or they might not grow in a full double coat to begin with. Your dog may be healthy, so take him to a veterinarian for a check-up. They will be able to tell you if he is truly under-weight and why.
