do you mean like making a living dex in a newer game (let's say swsh for example) and transferring all the pokemon up from their respective games? i mean it's probably doable, but would require a lot of work. also not all pokemon introduced in a generation are available in its region necessarily, like some gen 2 pokemon are not available until the kanto part of the game. so for example, houndour is not available in the "region" we associate it with, as it's only in kanto, not johto. unless you mean you want to catch them in the game they were originally obtainable in and not specifically the region. unless you hatch them...that's an option, i suppose.