Actionscript 3 writing a XML element inside a another with code
<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
</xmlcontainer>

function appendXML():void{

var xmlData:XML = new XML();
var xmlrequest:URLRequest = new URLRequest(String("xml/clientelist.xml"));
xmlLoader.load(xmlrequest);
xmlLoader.addEventListener(Event.COMPLETE, LoadXML);

function LoadXML(e:Event):void
{
        xmlData = new XML(e.target.data);
        xmlData.appendChild(<pessoa/>);
    xmlData.appendChild(<id/>);
    xmlData.id.appendChild(idfield.text);
    xmlData.appendChild(<nome/>);
    xmlData.nome.appendChild(nomefield.text);
    xmlData.appendChild(<email/>);
    xmlData.email.appendChild(emailfield.text);
    xmlData.appendChild(<contacto/>);
    xmlData.contacto.appendChild(contacto1field.text);
        trace(xmlData);

    var fileb:FileReference = new FileReference;
    fileb.save( xmlData, "clientelist.xml" );
}

var xml:XML = <a/>
xml.appendChild(new XML("<b>hello</b>"))
trace(xml.toXMLString());

<a><b>hello</b></a>

function addClient(xml:XML, name:String, phone:String):void {
 // will replace into tpl replace {name} and {phone} by their values
 var tpl:XML=<client><name>{name}</name><phone>{phone}</phone></client>;

 // append the new node to the xml
 xml.appendChild(tpl);
}

// test

var myXML:XML=<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
</xmlcontainer>;

addClient(myXML, "foo", "12345678");

trace(myXML.toXMLString());
// output:
<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
  <client>
    <name>foo</name>
    <phone>12345678</phone>
  </client>
</xmlcontainer>

var xmlData:XML = new XML(<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
</xmlcontainer>
);
xmlData.appendChild(<pessoa/>);
xmlData.appendChild(<id/>);
xmlData.id.appendChild('idFieldText');
xmlData.appendChild(<nome/>);
xmlData.nome.appendChild('nameFieldText');
xmlData.appendChild(<email/>);
xmlData.email.appendChild('email');
xmlData.appendChild(<contacto/>);
xmlData.contacto.appendChild('phone');
trace(xmlData);

// output is
<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
  <pessoa/>
  <id>idFieldText</id>
  <nome>nameFieldText</nome>
  <email>email</email>
  <contacto>phone</contacto>
</xmlcontainer>

// And to build on it dont forget to add CDATA tags to all user input fields.<br/>
var xmlData:XML = new XML(<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
</xmlcontainer>
);
var userID:String = '123456789'
var userName:String = 'John doe'
var email:String = 'my@email.com'
var phone:String = '888-555-1212'

xmlData.appendChild(<pessoa/>);
xmlData.appendChild(<id/>);
xmlData.id.appendChild( new XML( "<![CDATA[" + userID + "]]>" ));
xmlData.appendChild(<nome/>);
xmlData.nome.appendChild( new XML( "<![CDATA[" + userName + "]]>" ));
xmlData.appendChild(<email/>);
xmlData.email.appendChild( new XML( "<![CDATA[" + email + "]]>" ));
xmlData.appendChild(<contacto/>);
xmlData.contacto.appendChild( new XML( "<![CDATA[" + userID + "]]>" ));
trace(xmlData);

//output is
<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
  <pessoa/>
  <id><![CDATA[123456789]]></id>
  <nome><![CDATA[John doe]]></nome>
  <email><![CDATA[my@email.com]]></email>
  <contacto><![CDATA[123456789]]></contacto>
</xmlcontainer>

// to expand farther and clean up

var xmlData:XML = new XML(<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
</xmlcontainer>
);
var userID:String = '123456789'
var userName:String = 'John doe'
var email:String = 'my@email.com'
var phone:String = '888-555-1212'



var client:XML = new XML(<client/>)
//client.appendChild(<id/>);
client.appendChild( new XML( "<id><![CDATA[" + userID + "]]></id>" ));
client.appendChild( new XML( "<nome><![CDATA[" + userName + "]]></nome>" ));
client.appendChild( new XML( "<email><![CDATA[" + email + "]]></email>" ));
client.appendChild( new XML( "<contacto><![CDATA[" + userID + "]]></contacto>" ));

xmlData.appendChild(client);

trace(xmlData);

// output is
<xmlcontainer>
  <client>
    <name>Marco</name>
    <phone>123456789</phone>
  </client>
  <client>
    <name>Roberto</name>
    <phone>987654321</phone>
  </client>
  <client>
    <id><![CDATA[123456789]]></id>
    <nome><![CDATA[John doe]]></nome>
    <email><![CDATA[my@email.com]]></email>
    <contacto><![CDATA[123456789]]></contacto>
  </client>
</xmlcontainer>