var ary = ["a.xml","b.xml","c.xml"];
var len = ary.length;
var channel_count = 0;

for(var i = 0;i < len;i++){
  loadXMLFunction(ary[i]);
}

function loadXMLFunction(str){
  var tmp_xml = new XML();
  var funcOwner = this;

  tmp_xml.ignoreWhite = true;
  tmp_xml.load(str);
  tmp_xml.onLoad = function(success){
    this.parseXML_data(this);
  }
}

function parseXML_data(node){
  var tmp = new Object();

  //ここはデータにあわせてセット
  tmp["title"] = ...

  _level0["channel_"+channel_count++] = tmp;
}

<title>タイトルだよ</title>
<link>http://www.####.html
</link>
<description>Summary for """"</description>
<language>ja</language>
↓↓↓
<header title='タイトルだよ' link='http://www.####.html' description='Summary for """"' language='ja'>

これを上記のparseXML_dataで処理したら･･･

var atr_ary = ["title","link","description","language"];
var ary_len = atr_ary.length;
var obj = new Object();

for(var i = 0;i < ary_len;i++){
  obj[atr_ary[i]] = node.attributes[atr_ary[i]];
}