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Completely Possible
Lets say there is a Articuno someone has in Leaf Green that they want to finish their dex in X/Y
First they make sure the Articuno is in their PC, and then they save the Leaf Green Game.
They then fire up platinum and select "MIGRATE POKEMON" and choose the articuno, along with other pokemon then they go to the PalPark in Platinum and run around until they find the Articuno they wanted to catch and nab it.
With the Articuno safe in Platinum, it gets put into the PC again and then you the person saves the game.
They then go grab another DS and a copy of the 5th gen games. In the game they go to the pokemon transfer and link the two games together, and select the articuno for capture.
they then play that oddly fun crossbow game where they grab pokemon, and get the articuno.
Then said person either talks to someone who got lucky and got pokebank on a jap DS before it was taken off, or they wait for pokebank to finally come out after Nintendo gets its shit together and transfer it up to gen 6. they also then find out the articuno has no good IVs, isnt competitivly viable and that they just should have pokegened one in gen 5.