You cant because I tried. But you might be able to if you find a cheat engine code.
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Steal Trainers Pokemon F14e5bf8 48e23757 8e883eff 92e9660d
You cannot steal Pokemon but if you have a gameshark you can put it into your GBA (Gameboy Advance) and enter in a code for stealing the rental Pokemon.
00394721-34280
they went to a better world
Get it back. People shouldn't steal.