GTC Biotherapeutics (fka GTCB)

[used to make chicken]

What you are proposing would be significant, but represents an enormous amount of work. Point mutations and multiple nuclear transfers would be simple, cheaper, and less time-consuming. Beyond the initial development of said strains, you would still need multiple generations of cross just to get back to the homozygous background you created.

On top of this, such changes to the host animal would have an impact on, at least, the majority of proteins in milk, which in turn could lead to major hassles in purification schemes.