Create A Xml Document And Load It Into A 3rd Party Function?

I am trying to create a xml document and load it into a 3rd party function as follows:

var doc = new ActiveXObject('Microsoft.XMLDOM'); // OR
var doc = document.implementation.createDocument('', 'xml', null);
o.overlayKML('somefile.xml');
// Works perfectly
o.overlayKML(doc);
// Doesn't work at all, return unsupport error

I get to know that the overlayKML (3rd party function) needs to read a physical file with a path and sadly it doesn't support DOM. How can I create a javascript document that mimic a physical file and introduce it into the function?