If this was my 'mum pup', I would be very reluctant to be adding anything different to her diet while she is still feeding babies. You have no idea how this change is going to not only affect mum - but also babies.
The vet has said that she looks great, so I would take that comment on board as being correct. I am presuming this is the same vet that you have been using all through this time.
So does she really need the extra food ?