If you mean "how can I get Johto Pokemon into Pokemon Emerald", then you can do that by either catching them in-game (as far as I remember) after completing the main Pokedex.
The three Johto starters can be obtained once you complete (catch) all Pokemon in the regional Pokedex. You can also transfer their second (or third-) stage evolutions from Pokemon Colosseum via GameBoy Advance-Nintendo GameCube link cable. You need to finish the story in Colosseum for this to work, however.
If you mean "how can I transfer Pokemon from HeartGold/SoulSilver/Gold/Silver/Crystal to Emerald", put simply, you can't. Sorry.
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You get one Johto starter in Emerald for completing the Hoenn Pokedex. You can trade them into FireRed. Unfortunately, Croconaws (along with Totodiles) cannot be caught in Pokemon FireRed. You must have one traded to you from Emerald.
Basically, -just- and I use the term loosely, obtain all Pokemon obtainable in Emerald/ruby/sapphire, on such games, then choose a johto starter (Birch will call you) and trade. Only way, sorry.
It's possible yeah.
You have to complete your Hoenn Pokedex. After you get the diploma go to a PC at and youll find a Johto pass. Go on the Ferry and they'll take you to the Johto region. I don't know if this works; I haven't tried it yet. My friend told me it ONLY works for Emerald.
In Pokemon Emerald, it's impossible to leave the Hoenn Region. The Kanto, Johto, and Sinnoh Regions are inaccessible in Emerald. Sorry.