Hi Agni,
thx for your hint, it brought me to the solution. For those who wanna do the same here a short snippet:
XDocument newForm = thisApplication.XDocuments.NewFromSolution("http://Server/Web/test.xsn");
IXMLDOMNode node = thisXDocument.DOM.selectSingleNode("my:myFields...");
IXMLDOMNode testNode = newForm.DOM.selectSingleNode("my:myFields....");
testNode.text = node.text;