I need to access external config files within my custom sitemap.xmap -- either forrest.properties or skinconf.xml.

  <map:pipeline>
   <map:match pattern="my_pattern.xml">
    <map:generate src="my.xml"/>
    <map:transform src="{project:resources.stylesheets}/my-to-doc.xsl">
        <map:parameter name="my_parameter_for_my_xsl" value="xxx"/>
    </map:transform>
    <map:serialize type="xml"/>
   </map:match>
  </map:pipeline>

What should I have instead of xxx in order to get parameter from either forrest.properties or skinconf.xml?

Thank you