buzz.xml

<?xml version="1.0" encoding="UTF-8"?>
<buzz>Buzz</buzz>

fizz.xml

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="style.xslt"?>
<fizz>Fizz<buzz/></fizz>

style.xslt

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="/">
    <root>
      <xsl:apply-templates/>
    </root>
  </xsl:template>

  <xsl:template match="buzz">
    <xsl:value-of select="document('buzz.xml')"/>
  </xsl:template>


</xsl:stylesheet>