When can you trade Johto Pokémon in Pokémon Emerald?

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.