Develop an API to convert a JSON client request to xml and vise versa in SoapUI

Given a hosted back end service I need to develop an API. Is there a way to use the WSO2 Application Server product to develop the API and use SoapUI to develop that API that can take a client request which is given in json format and convert it to a xml format and vise versa. I am new to API creation and an explanation with detailed steps will be much appreciated. 

Thanks in advance.