[keycloak-user] How to pass an arbitrary user name into the access token granted by Keycloak?

Our RESTful services configured with bearer-only access-type are used also by third-party application with their own user realm - only proprietary accessible by the third-party application. There is the requirement, to audit the user of the third-party application, invoking our RESTful services by the RESTful service itself - so it would need to be passed into the JWT bearer token which granted by Keycloak to the third-party application. Is there another way as importing the user-realm of the third-party application into the Keycloak realm associated with our RESTful service and the third-party application as client, so the third-party application can pass the user id by a resource owner password credentials authorization grant to obtain the access token - which relies on keeping our copy of the user-realm in sync with the third-party application, and that the third-party application has access to the user password. Particularly, is it possible to use a Bearer JWT as an authorization grant according RFC 7523, Section 2.1 to pass the user id from the third-party application to Keycloak, and to configure Keycloak to pass the user id also in the returned JWT Access Token? Thanks, Gunter