Use fluentd with logback

Hi team,
I would like to use the standard fluentd deployment that comes with GKE, but I am not sure whether it is possible or not.
I've got a bunch of Java applications writing logs into fluentd via logback appender, but the problem is that I am not sure how to connect the appender to that particular fluend.
Of course capturing std outputs and writing into stackdriver is working but that option is pretty limited since I need to tag messages and add additional fields.

So, question: is there any way to connect the appender (or any other app that connects to fluentd via host:port) to this built-in fluentd in a GKE cluster?

Sorry if it is not the right place to ask this question.

Thanks