Relic Castle

Okay, so I'm trying to build another one of those games where the player plays as Team Rocket, and I have a question about the "snag ball" feature.

How exactly would I go about making a Poke Ball that turns any Pokemon caught with it into a shadow Pokemon?

I already have a good idea of how to make a "snag ball" that will allow catching of other trainer's Pokemon, but I also wanted the Poke Ball to turn the Pokemon into a shadow Pokemon upon capture.