I start putting food in for my puppies about 4 - 5 weeks of age. They will need to keep suckling from mum, but there is food there for them when she is out of the whelping box.
Here is a link for something you might try. Be sure to read the 1st 8 posts in this thread, particularly the answers that Yogi has given! He used Purina but you could use any brand of puppy food you prefer--just stay away from cheap crappy dog food please!---->