Accessing request-param in XML

[321los]

Hello community,

my sitemap:
<map:match pattern="zipped.zip">
         <map:generate src="">            <map:parameter name="pk" value="{request-param:pk}"/>  
         <map:serialize type="zip"/>
 </map:match>

my zip.xml
<zip:archive xmlns:zip="http://apache.org/cocoon/zip-archive/1.0 ">
  <zip:entry name="print.pdf" src="" href="http://localhost:8080/cocoon/print.pdf?pk=xx">http://localhost:8080/cocoon/print.pdf?pk=xx">
  </zip:entry>
</zip:archive>


What I want to do:
call the URL http://localhost:8080/cocoon/zipped.zip?pk=45. The request parameter should be passed to zip.xml and be put into the src URL of my print.pdf; so that the zip.xml looks in this case like that:
<zip:archive xmlns:zip="http://apache.org/cocoon/zip-archive/1.0">
  <zip:entry name="print.pdf" src="" href="http://localhost:8080/cocoon/print.pdf?pk=45">http://localhost:8080/cocoon/print.pdf?pk=45">
  </zip:entry>
</zip:archive>

(Something like the SQLTransformer '<sql:substitute-value sql:name="pk"/>')
How can I achieve that?

Thanks
Marco