Originally to use expansions you would pick them in your Poké Gear or Radio (whatever that is), but in my head I think it would make more sense to make expansions accessible from a new item, making it differate from the Pokémon games giving it a little bit of originality, but not making it something crazy you'd never see in a Pokémon game.
I think of it as a item called Wardrobe, your mom would give it to you in the beginning of the game, going under the Key Items section of your bag, and it allows you to change your "look" by applying expansions to your character.
