I did miss your point! If they are adding artic eel genes to pacific salmon, how could that ever happen in nature?
Randomly. Any finite number of items (4 genome nucleotides G,A,T, and C) assembled in a finite sequence (one chromosome) has a finite number of possibilities. An unimaginably large number of possibilities, but still finite. Genes mutate randomly (lightning strikes, radiation events, etc)so any possible sequence can happen naturally, mathematically speaking. So you can't say that it would "never" occur, it's always technically possible. It's just that you might have to wait a very long time for it.
Now that line of thinking has been beat into the ground..... It was actually supposed to be lighthearted and funny.