If I got your question right, the only way that I found so far is:
-- switch to the Projects pane in ReadyAPI and select the request that you need;
-- In the Request Editor set value of the Media Type combo to 'application/json'. This will enable the edit field below where you can enter the data that you need.
Note, that request body and entered data are available from script during simulation (https://community.smartbear.com/t5/Service-Virtualization-Virtual/Extracting-value-from-requestBody-...)