Note the use of <xsl:output/>, the declaration of other namespaces, and the use of <xsl:copy-of/>
Code: ( text )
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:npl="http://www.epo.org/npl" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<xsl:output method="xml" doctype-system="npl-entities.dtd"/>
<xsl:template match="/">
<nplbiblio>
<xsl:attribute name="xsi:noNamespaceSchemaLocation">npldoc v2.0.xsd</xsl:attribute>
<xsl:attribute name="rundate">20080123</xsl:attribute>
<xsl:for-each select="nplbiblio/document">
<document status="n">
<xsl:attribute name="creadate">20080123</xsl:attribute>
<xsl:copy-of select="xp"/>
<attribs>
<attrib name="doi-art">
<xsl:value-of select="doi"/>
</attrib>
</attribs>
</document>
</xsl:for-each>
</nplbiblio>
</xsl:template>
</xsl:stylesheet>