[ulik]

wvxvw я вас не совсем понял... Что-то типа такого вы хотите:

package
{
	public class LimitedXMLList 
	{
		private var _data:XMLList = new XMLList("<doc><node /></doc>");
		private var supportedNames:Array = [ "id", "login", "password" ];
		private var numNames:int = supportedNames.length;
		private var hasValue:Boolean;
		
		public function LimitedXMLList()
		{
			super();
		}
		
		public function addParam(varName:String, varValue:String):Boolean
		{
			hasValue = false;
			for(var i:int = 0; i<numNames; i++)
			{
				if(supportedNames[i] == varName)
				{
					hasValue = true;
					break;
				}
			}
			if(!hasValue)
			{
				throw new Error("Unsupported varName = '"+varName+"'. Supported names: "+supportedNames, 0);
				return false;
			}
			_data.node..@[varName] = varValue;
			return true;
		}
		
		public function get data():XMLList
		{
			return _data;
		}
	}
}

import LimitedXMLList;

var myList:LimitedXMLList = new LimitedXMLList();
myList.addParam("id", "659");
myList.addParam("login", "Ivan");
myList.addParam("password", "test");
myList.addParam("someVar", "someValue"); // Error: Unsupported varName = 'someVar'. Supported names: id,login,password
trace(myList.data);

После первых трех add'ов:

<doc>
<node id="659" login="Ivan" password="test"/>
</doc>