Integrate existing AWS Cognito user pool into iOS project with Amplify

Amplify CLI doesn't support existing resources at the moment but you can manually edit your awsconfiguration.json to include the User Pool that is already present as outlined here.

Update: Since 2020 Amplify allows you to use existing User Pools (yeah!). Just run:

amplify import auth

Source: https://aws.amazon.com/de/about-aws/whats-new/2020/10/use-existing-cognito-user-pools-identity-pools-for-amplify-project/