How can we edit existing JSON file and ad Some additional Fields to the existing format using talend .
As a JSON file is just plain text, you can't easily/reliably do any processing directly on this, so you'll need to read it in using a tFileInputJSON, do whatever manipulation you require, e.g. add new fields, and then write it back out with a tFileOutputJSON.
Introduction to Talend Open Studio for Data Integration.
Practical steps to developing your data integration strategy.
Create systems and workflow to manage clean data ingestion and data transformation.