How to construct a new payload using the existing payload?

Adding existing payload as a child element!

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Existing Payload

<?xml version="1.0" encoding="UTF-8"?>
 <soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope">
    <soapenv:Body>
       <order>
          <price>12</price>
          <productid>IC002</productid>
          <quantity>2</quantity>
          <reference>ref</reference>
       </order>
    </soapenv:Body>
 </soapenv:Envelope>

Now we want to construct a new payload using the existing payload body as the child element. We get the <order> element as a child of <orders> element in the new payload.

New Payload

<soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope">
   <soapenv:Body>
      <orders>
         <order>
            <price>12</price>
            <productid>IC002</productid>
            <quantity>2</quantity>
            <reference>ref</reference>
         </order>
      </orders>
   </soapenv:Body>
</soapenv:Envelope>

We can use the WSO2 Enrich mediator in order to achieve this.

<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse"
       name="1_6_1_3_Proxy_addCurrentPayloadAsChild"
       startOnLoad="true"
       statistics="disable"
       trace="disable"
       transports="http,https">
   <target>
      <inSequence>
         <enrich>
            <source clone="true" type="body"/>
            <target property="originalPayloadBody" type="property"/>
         </enrich>
         <enrich>
            <source clone="true" type="inline">
               <orders xmlns=""/>
            </source>
            <target type="body"/>
         </enrich>
         <enrich>
            <source clone="true" property="originalPayloadBody" type="property"/>
            <target action="child" xpath="//orders"/>
         </enrich>
         <respond/>
      </inSequence>
      <outSequence/>
      <faultSequence/>
   </target>
   <description/>
</proxy>

For further reading please refer:

WSO2 Integration Studio

WSO2 Enterprise Integrator