It's not possible. Games are never programmed with the data relevant to any Pokemon introduced in later generations. Even if you somehow could break Poketransfer and send the Kalos Pokemon back, it would likely get rejected by the game as an illegal mess of code or corrupted Pokemon data.
The only way you could get Kalos mons into an Unova or earlier game would to be to take a ROM and edit the game data to create versions of the new Kalos mons. But not only would that be extremely difficult, more trouble than it's worth, and utterly irrelevant for anything other than the novelty, it'd also most likely be several kinds of illegal (real, law-breaking illegal, not "the game doesn't allow this" illegal).
