It is possible to integrate your legacy application with Auth0 using a combination of approaches, and there are out-of-the-box solutions available:
Migrating Users to Auth0
- Auth0 offers strategies for migrating users from an existing application, such as bulk importing users via the Management API or the User Import/Export Extension Technical Strategies for Migrating Users to Auth0
Partial Migration to Auth0
- If you want to keep your existing authentication pipeline and gradually move parts over to Auth0, you can create newer parts of your application with a .NET CORE API using Auth0 and move parts over piece by piece Question about implementation of legacy applications
Integration via M2M Token
- To give access to users from the legacy application to the Auth0 managed service without requiring an additional login, you can consider using a Machine-to-Machine (M2M) token for this application to generate a token to give to the user. Once redirected to the service, the user would not need an additional login
there are methods available to integrate your legacy application with Auth0 and provide access to users through the Auth0 managed service without requiring them to be set up in the Auth0 database. You can leverage strategies such as partial migration and M2M tokens to achieve this integration.