For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there aren't any extra letters Later on and thus needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however you would've utilised “addi” for “addict”. BIP39 and http://remingtonjszi611.almoheet-travel.com/from-words-to-wide-range-how-to-efficiently-use-bip39-passphrases-for-your-copyright-wallets