OK dude. You need AR (Action Replay) codes for that. I have HeartGold, but you know, HeartGold isn't that different from SoulSilver. And one more thing- I don't think its nice to steal Pokemon or anything else- that's what the game says when you throw a Pokeball at a trainer's Pkemon (''Don't be a thief!'')
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i don't know but sertch on Pokemon cheats.com
If you mean another trainers Pokemon you have to use cheat codes
9223b5fa 00002101 1223b5fa 00002100 d2000000 00000000
Unless there is a cheat code that allows you to, NO!The only games that you can catch other Trainers' Pokémon in are the GameCube games Colosseum and XD: Gale of Darkness. Even in those, you can only catch certain Pokémon from other Trainers.
to get data on Pokemon in your pokedex, capture the Pokemon unless they are trainers Pokemon then you have to cheat to get trainers Pokemon.