[keycloak-user] Federated Identity linking custom attribute

Hello, When using a SAML Identity Provider to link to existing keycloak users. Keycloak uses the NameID to do the linking. How can I use another SAML attribute to do the linking? I have tried the "Username Template Importer" in the identity provider, but this only works when creating new users. In my case the user is already created and I just need to do the linking. Thanks in advance for your help, Daniel