convert file to .xml?

If you were to convert it to .xml in TextEdit, you would have to make sure that it is in Plain Text Format (Which you can do by clicking on "Format" and then "Make Plain Text"). Now when you go to save your file, save it with a Plain Text Encoding of "Western (Mac OS Roman)" and at the top make your extension .xml (e.g. index.xml)

It depends on what program you're using. My guess is that you can download a utility for converting text files, but as a starter, you can convert a text file in TextEdit to Word XML.