It is not possible to combine existing accounts into a family account. If you want to share your account with someone who already has their own account, one of you will have to give up their history and book lists.

Choose which account you will keep using and cancel the subscription on the other account.

Once the cancelled account has run out, you can add a profile to the active account. Now the user of the new extra profile only has to log out of the app and in again with the family account credentials.