The rules of Unseen Forces - Expansion setup says to 'add the investigator cards to the investigator deck', 'add the new ancient one cards the ancient one deck' ... But for the new adventure cards the rules say to 'shuffle cards into their deck'. Same for the new Mytos cards, the new common item cards, new unique item cards, ally cards: it's also 'shuffle into their respective decks.
Not sure why 'shuffle into' is used instead of 'add'
Concrete question: should the new common item cards, new unique item cards, new Ally cards and new Mythos cards be added to the respective decks of the base game. Or should I only use the new cards when playing the expansion