Suppose we wish to replace 27th CD with:

<cd27 pos27="Armstrong">...</cd27>   New element/attribute dependent on input
         

<xsl:template match="cdlist">
  <xsl:copy>
  <xsl:for-each select="cd">
    <xsl:sort select="artist" />
    <xsl:element name="{concat('cd',string(position()))}">
     <xsl:attribute name= "concat('pos' , string(position()))"
       <xsl:value-of select="title" />
     </xsl:attribute>
     <xsl:apply-templates />
    </xsl:element>
  </xsl:for-each>
  </xsl:copy>
</xsl:template>