Not unless you have an action replay plugged into the back of your DS.
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You can usually catch others Pokemon by using a Master Ball(does not always work)
Other than the masterball try using a lot of ultra balls.
First, give a masterball to a Pokemon (suggestion: legendary,) then put it in an empty box. When it is finished doing the "Saving... do not turn off the power" thing without u pressing A, shut it off. when u turn it back on, it should have your Pokemon in your party, and your masterball in your bag. the Pokemon u copied will also be in your PC and it will be with the masterball!!! this cheat also works on other versions
no but if you catch all the hoenn pokemon you can catch jotho and kanto pokemon
You are not allowed to catch other trainer's pokemons on any of the Pokemon games. You may only catch Pokemon from the wild or receive them through other means (events, game corner, eggs, etc.)