Well for me personally, we have all these editors out there, we surely do have all those Pokémon in there, but the way I see it, extra options doesn’t hurt, it adds a challenge for me.
There is a problem though, Pokewalker data isn’t saved like normal game data is, therefore the data for the Pokewalker will reset each time the game is restarted, in his case, his save will still act as a clean slate as if he never used a Pokewalker, UNLESS I personally edit his save manually, which I can also do. Yes codes can make this work, but since the data can’t be saved, it would be partially pointless, that’s why manual is the way to go.
Edit:
Just did some testing, doesn’t matter with a cheat code or manual typing, the save doesn’t want to take the modifications without corrupting the save (resulting in it using your last save point), there must be some type of check between the Pokewalker device and the game where it just doesn’t allow any modifications to the that section of the save, could also be a checksum not matching up with the data at hand. Either way, I would just avoid editing the Pokewalker data altogether.