Can I steal other trainers' Pokemon?

No, you cannot steal other trainers' Pokemon. If you try to (you throw a Pokeball at the Pokemon), the trainer will block it. If you want what they have, you just have to raise your own!

No, you cannot steal the Pokemon of other trainers. They have been rightfully captured by them, so cannot be re-captured by you. As the protagonist of the game (and automatic hero), the game objects (mainly on a moral ground) when you select the action to throw a PokeBall during battle. The trainer blocks your action, preventing the capture of their Pokemon, resulting in a loss of a PokeBall and no gain of a Pokemon.

I'm sure that if you get the trainer out of the way, you can steal their Pokemon. Perhaps there is a cheat code available that allows you to steal.