how to insert headers in the event

Hi,
i want to change my current setup (log4j with syslog appender => syslog-ng) to log4j with flume appender => flume-ng

i want to insert several headers to the avro event with log4j but i don't find how to do this.
Everything i know is how to customise the body with the ConversionPattern.

so how can i directly insert header to the event so that i can route event to different sinks bases on them ?

thanks
Jean