Can I feed my nursing dog puppy milk re-placer to help her get stronger?

This can be potentially fatal for mum dogs. It is recommended that the "breeder" wean the pups off mum and feed the pups by hand. Yes it is hard like you've already found out. You can use a syringe to get the milk in the pups. If htey genuinely are not feeding then you dont wait till the vet is open you pay for an emergency visit because pups can and often do die VERY quickly in the first few weeks due to lack of nutrition.