Well, looking at that tutorial, you'd need to:
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Add your new field to the CustomListCAML.xml file
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Modify the code where it loops through the repeating table and adds items to the CAML Batch XML
Or you could just try qRules. There is a free trial available, and here's a video showing you the steps. To top it all off, with qRules you can get back the id of the list item, so that you can easily update from within your InfoPath form.