Using both the API Manager and the WSO2 ESB, you can implement an API facade architecture pattern. WSO2 recommends this architecture if you are performing heavy mediation in your setup. For implementation details of an API facade, see implementing an API facade with WSO2 API management platform. Since the API Manager does not have the ESB's GUI to perform mediation functions, you need to use the XML-based source view for configuration. Alternatively, you can create the necessary mediation sequences using the GUI of the ESB, and copy them from the ESB to the API Manager.
Also see the following usecases in WSO2 ESB documentation for more information on REST to SOAP conversion.
Although the API Gateway contains ESB features, it is recommended not to use it for ESB-specific tasks. Use it only for Gateway functionality related to API invocations. For example, if you want to call external services like SAP, use a separate ESB cluster.