PowerCenter does not support JSON. You may be able to use java to create but nothing natively inside of PowerCenter.
If you have access to IICS and Data Engineering you can do this.
i can only second Scott's advice to look for a Java transformation, that's probably the best way in PowerCenter.
Please search this forum (the PowerCenter forum) for the keyword JSON, you should find at least one thread where some sample code was posted; you can use this as a starting point.