Signs on the Sand

I'm missing something obvious and spent already about two hours on that simple problem. I hope somebody profficient in MSBuild drops me a line. How do I build MSBuild custom task that has XML subtree?

Here is my NAnt task:

<nxslt in="books.xml" style="books.xsl" out="out/catalog.html">
  <parameters>
    <parameter name="param1" namespaceuri="" value="val1"/>
  </parameters>
</nxslt>		

How do I build custom MSBuild task that accepts such <parameters> subtree??? The documentation on MSBuild sucks. I mean it's fine if you just using tasks, but if you want to build your own task you screwd up.