Integrating an external login management to existing Knack app

I wish to implement an external Login account management to allow our users to have access to their account on our Knack app while also allowing us to manage their account details in a MariaDB MySQL database we host.

Knack suggests using SSO but I have no knowledge or experience on how to go about this. I am a retired volunteer developer for Ecobricks.org and whatever we use has to be open source. In my working life I was a RDBMS developer and typically the SSO was kerberos and entirely managed by other teams. I was just a user.

We want to be able to migrate the users we have (~79K) without their logins failing because we don’t know their password. We obviously cannot just export the passwords but how can we validate them via API?
Any suggestions of open source products to investigate or methods to explore would be gratefully received.

Richard Goldsmith