If you were hoping to use your level 100 Charizard from the eShop version of Pokémon Red and Blue to take down Alola's Island Challenges, you'll have to wait a bit. According to the official Pokemon site, the Pokémon Bank won't become compatible with Sun and Moon until January 2017.
First released alongside Pokemon X and Y, the Pokémon Bank is a 3DS app that enables you to store and move large numbers of monsters at once. Working in conjunction with the Poké Transport app, it currently enables the transfer of monsters from Pokémon Black and White. The app carries with it a $4.99 annual subscription.
When the update drops next year, it will connect Sun and Moon with much of the rest of the series, including eShop versions of Pokémon Red and Blue. But if you were hoping that monsters would be able to be freely transferred back and forth between Gen 6 and Gen 7, Game Freak has some bad news for you.
"If you transfer a Pokémon to Pokémon Bank and then on to Pokémon Sun or Pokémon Moon from Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X, or Pokémon Y, you’ll be unable to transfer it back to Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X, or Pokémon Y," the official site explained.
What a shame. This is in addition to the usual restriction on transferring items from game to game.
Happily, while the news post only mentions Gen 5 games and beyond, it will still be possible to transfer monsters from games as old as Ruby and Sapphire all the way up to Sun and Moon. If you have a Nintendo DS and the requisite copies of Pokémon, you can transfer them from the Ruby/Sapphire/Emerald generation to the Diamond/Pearl/Platinum generation, and on to Black and White. From there, you can move them to Sun and Moon. So if there are any hard-to-find legendary monsters you're missing, that's how you can get them.
That won't be possible until January, though, so you'll have to make do with the monsters available in Sun and Moon until then. No doubt this was Game Freak's intent, as it will give the Alolan Pokedex a chance to shine in the meantime.
Sun and Moon is now available on the Nintendo 3DS, and you can read our review here. Additionally, I've got a journal going that is covering the series from a hardcore perspective; and if you have questions about the game itself, we've got some guides for you.
Now back to waiting impatiently for January.