There are a few ways of doing this. Some database programs like Excel will let you output in XML format. The other option is to write a script or application to do the job. We have some web based applications that create the XML schema and then add the data to the file. This is then is pass to a web service for processing.
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What script/ programming language are you using now. Did you hint for xml,etree using python. If this is OK, please link here for more information: https://docs.python.org/2/library/xml.etree.elementtree.html#supported-xpath-syntax