Yes,it's possible with record oriented processors(Convert Record) NiFi,
1. Consume Kafka Processor //to read data from Kafka
2. Convert Record processor //to convert json format to csv format
3. PutHDFS //write the csv format data to HDFS
In ConvertRecord processor we are configuring Record Reader(Json Tree Reader) to read incoming Json format data and Record writer Controller service(CSVSetWriter) to write the output flowfile in CSV format, we need to define Avro schema for the Record Reader/Writer controller services.
Follow this tutorial to know more details and configurations regarding Convert record processor.