Found a strange glitch where if you add an item that you don't have you can't register it into the battle pouch.
Edit: looks like obtaining the item legitimately lets me use register it into the battle pouch while retaining the edited number, so maybe there's another variable at play here.
I have had cherry "pizza" before basically just a pizza crust with cherry pie filling poured on it and a frosting drizzled on top. Delicious but I would hardly call it a pizza.