You can achieve this by adding an operation policy to the API with Payload Factory mediator. You can process and transform the json content in your payload using this mediator on the response path. Refer the following to get an idea about this.

[1] - https://apim.docs.wso2.com/en/latest/design/api-policies/regular-gateway-policies/transforming-api-message-payload/

[2] - https://ei.docs.wso2.com/en/latest/micro-integrator/use-cases/examples/json_examples/json-examples/#constructing-and-transforming-json-payloads