Jasper SubReport with XML-Document from JDBC-String-Column?

i'm trying to create a report with a subreport from a jdbc table where one string column contains a XML-Document which should be the datasource for the subreport.

I have created the sub-report with an JRXmlDataSource and pass it like:

<dataSourceExpression><![CDATA[new net.sf.jasperreports.engine.data.JRXmlDataSource(new java.io.ByteArrayInputStream($F{xmldata}.getBytes()))]]></dataSourceExpression>

from the Main Report.

In the subreport i get a REPORT_DATA_SOURCE with correct type but no XML_DATA_DOCUMENT. How would i now create the XML_DATA_DOCUMENT in the subreport to evaluate the queryString (language="xPath") in the subreport and display my fields?