You need a game shark
You need a game shark you can also get them from emerald but you have to complete the pokedex in emerald to get oe of the three starters from pro. berch
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U get all the Pokemon you can't go to the johto region in pkmn ruby/Sapphire you can only do that in Pokemon 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.
Not any hoenn Pokemon but firered and leafgreen can catch alot of kanto and johto Pokemon ruby and sapphire can and can't.
Hoenn is the only accessible region in pokemon Ruby, Sapphire, and Emerald
All Pokemon from Kanto, Johto, and Hoenn can be obtained in Emerald. Susrskit, Masquerain, Meditite, Medicham, Roselia, Zangoose, and Lunatone must be traded from Ruby or Sapphire. Other Pokemon like all of the starters must also be traded over.