So how can I create the element <SubConcepts> below <Top> and not <PrimeConcept> ?

just move it there in the stylesheet, what's the problem?

incidentally

<xsl:element name="SubConcept">
<xsl:attribute name="id"><xsl:value-of select="@id"/></xsl:attribute>
<xsl:attribute name="name"><xsl:value-of select="@name"/></xsl:attribute>
</xsl:element>

is a very long way of writing

<SubConcept id="{@id}" name="{@name"/>