You need to use
Code:
<xsl:copy-of select="document('customerorder.xml')/*"/>
So start with the identity transformation template
Code:
<xsl:template match="@* | node()">
<xsl:copy>
<xsl:apply-templates select="@* | node()"/>
</xsl:copy>
</xsl:template>
then add a template for that Payload element that copies the other document in e.g.
Code:
<xsl:template match="ns1:Payload"
xmlns:ns1="http://eai.yash.com/eBiz/Envelope">
<xsl:copy>
<xsl:apply-templates select="@*"/>
<xsl:copy-of select="document('customerorder.xml')/*"/>
</xsl:copy>
</xsl:template>