You can use the <b:param> tag as follows:
<p:birt
xmlns:ui="http://java.sun.com/jsf/facelets"
xmlns:s="http://jboss.com/products/seam/taglib"
xmlns:p="http://jboss.com/products/seam/birt"
designType="run"
format="pdf"
designName="test.rptdesign"
title="JBoss Birt Test">
<p:param name="sample" value="sample param"/>
</p:birt>