There are no items or methods in the game that allow you to steal or forcefully capture other trainers' Pokemon.
The primary way to obtain Pokemon in Pokemon Pearl is through wild encounters and trading with other players.
Here's why you can't catch other trainers' Pokemon:
* Gameplay Mechanics: The game is designed around battling and challenging other trainers, not stealing their Pokemon.
* Story and Morality: It wouldn't align with the spirit of the game or the bond between trainers and their Pokemon to allow for forced captures.
It's important to play the game fairly and respect other trainers and their Pokemon!