If you want to POST JSON data to a webspeed program, check out WEB-CONTEXT:FORM-INPUT or if you post more than 32K, check out WEB-CONTEXT:FORM-LONG-INPUT.
Now... regarding reading the JSON data, it depends on your OpenEdge version. In 10.2B Progress started supporting JSON, however it is very limited, especially if you have little control of how the JSON gets created. Since you are the one creating the JSON data it may work for you. Version 11.1 has much better support JSON including a SAX streaming implementation.
We were on version 10.2 so I had to resort to using this C library to convert the JSON into a CSV file. If you have access to Python on your server it is very easy to convert to a CSV file