Guard dog, I didn't create the standards for a breed. Every purebred was a mixed breed. After the mixture of breeds they determined that a certain traits were acceptable to call it a separate breed. If has the be able to reproduce the same traits consistently in or to gain acceptance as a breed. This is why designer breeds like the golden doodle is not recognized as a breed in any large registry. No one has been able to consistently produce litter after litter with consistent traits. Some look more like Goldens and other look more like poodles. I imagine that one day they will gain acceptance. But that will take a lot of work and science to figure out what percentage of each it will take to produce the same look in every dog in the litter. They will have to determine size, color, tail, etc.
Once this is established you don't continue to add breeds. You use dogs from the foundation stock and litters produced by the stock and their litters as long as the litters are from this point one breed.
Adding dogs from outside the breed is not reproducing the same breed. If you are going to do that you don't need to use a breed registry. You just have a dog.