Forward Only Selected Fields

There is a need to forward logs to a Splunk server.
Is there any way to filter out fields from a stream output?
Is there any way to add additional fields to a stream and/or output (with constant value)?

He @Tiger

I do not know of any plugin where you can do the requested - What you could do, use the processing pipeline.
Create new messages from the ingested with only the fields you need and route that into a specific stream and forward that stream.

Sorry but that is the only option I can imagine.