Add XML node dynamically with groovy

I have an XML similarly like below

<root>
<item>
<refitemid>10</refitemid>
</item>
</root>
    

my requirement is first to add one node <additional_info> to , then add new node to <additional_info> base upon the fetched data so that the new XML would present as below

<root>
<item>
<refitemid>10</refitemid>
<additional_info>
<sn><id>1</id></sn>
<sn><id>2</id></sn>
</additional_info>
</item>
</root>
    

I tried to achieve this with below groovy script.

def oPayload = new XmlSlurper().parseText(message.getBody() as String); //get above original XML data
if (oPayload.root.item.additional_info == "" || oPayload.root.item.additional_info == null) {
    oPayload.root.item.appendNode {
        additional_info {}
    }
}
for (int i = 1; i < 3; i++) {
    oPayload.root.item.additional_info.appendNode {
        sn {
            id(i.text())
        }
    }
}

oPayload = XmlUtil.serialize(oPayload)
    

unfortunately, above code is not working at all, as the result(in oPayload) will become as below:

<root>
<item>
<refitemid>10</refitemid>
</additional_info>
</item>
</root>
    

Can you please share your idea how to achieve the requirement? thanks a lot.

If you use XMLParser, made it a bit easier in my opinion:

import groovy.xml.XmlParser
import groovy.xml.XmlNodePrinter

def xmlText = '''<root>
<item>
<refitemid>10</refitemid>
</item>
</root>'''

def root = new XmlParser().parseText(xmlText)
def item = root.item[0]

def additionalInfo = item.additional_info?.first() ?: item.appendNode('additional_info')

(1..2).each { i ->
    additionalInfo.appendNode('sn').appendNode('id', i.toString())
}

def writer = new StringWriter()
new XmlNodePrinter(new PrintWriter(writer)).with {
    preserveWhitespace = true
    print(root)
}

println writer.toString()