is it possible to load an Excel-XML-Sheet (*.xml) with Flexcel?
Sadly no, this isn't supported or on our todo list. Excel-xml was a short lived format, introduced in Excel 2003 and deprecated in Excel 2007 (to xlsx). But even if xlsx is also xml, it is completely different, so supporting Excel2003 xml would mean a lot of extra code, which we can't justify at least at the moment.
Excel 2003 Xml is basically a dead format, only kept alive by some 3rd party tools which create files in that format because they haven't been upgraded.