Considering maven usage, you could use maven profiles with properties and reference those in the application.yml

pom.xml

<project ...>
    
    <profiles>
        <profile>
            (...)
            <properties>
                <kafka.key>${SOME_KEY_STOREPWD}</kafka.key>
            </properties>
        </profile>
    </profiles>
</project>

application.yml

kafka:
  ssl:
    key-password: '@kafka.key@'

See also:

• Using Maven properties in application.properties in Spring Boot
• How to refer environment variable in POM.xml?