XML:
<document>
<element_A>
     textA textA textA
     <insert>element_B</insert>
     textA textA textA
</element_A>

<element_B>
     textB textB textB
     <insert>element_C</insert>
     textB textB textB
</element_B>

<element_C>
     textC textC textC
     textC textC textC
</element_C>
</document>

XSLT:
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
	
	<xsl:template match="document">
	<xsl:apply-templates select="*[1]"/>
	</xsl:template>
	
	<xsl:template match="insert">
		<xsl:variable name="element" select="."/>
		<xsl:apply-templates select="/document/*[name()=$element]"/>
	</xsl:template>
</xsl:stylesheet>

Cheers,
RH