To parse the json string and convert to IS document use the below service in WmPublic. To get the file content use getFile in the same package.
pub.json:jsonStreamToDocument
pub.json:jsonStringToDocument
Once you have the IData document you can easily continue from there. Please note that the JSON support was added from wM 9.0.
What is your wM version?
#webMethods#Integration-Server-and-ESB