Unless your question really is about logstash-forwarder (which is depreacted, as Mark says) I suggest you edit your post and move it to the Logstash category. Quick answers in the meantime:
There's no such built-in conversion, but the json filter or codec can parse your JSON into fields and you can write a small piece of Ruby code in a ruby filter to convert those fields into key/value pairs.
Apache, Apache Lucene, Apache Hadoop, Hadoop, HDFS and the yellow elephant
logo are trademarks of the
Apache Software Foundation
in the United States and/or other countries.