Bob,
That is not totally correct, for two reasons. 1) The XML export doesn't contain as much information as the .bsmx export (Target Mash pH is one example), 2) some of the calculated values are not in the .bsmx export. There is enough information there that BeerSmith can import the file and re-create the calculated values, but not every value that appears in a BeerSmith window is explicitly included in the exported file. Example: Mash Efficiency is not listed explicitly in the XML or .bsmx file, but you can calculate it from other values that do appear in the exported files.
I am not aware of any way to add fields to the XML export.
--GF