The following expression should work:
Exslt.ExsltStrings:lowercase(concat($url, umbraco.library:NiceUrl(@id), '/', $AppendedID, '/'))
Testing with the following piece of code ....
<xsl:for-each select="$currentPage">
    <xsl:variable name="url" select="'http://www.EXAMPLE.com'" />
    <xsl:variable name="AppendedID" select="123" />
    <a href="{Exslt.ExsltStrings:lowercase(concat($url, umbraco.library:NiceUrl(@id), '/', $AppendedID, '/'))}">
        <xsl:value-of select="@nodeName" />
    </a>
</xsl:for-each>
.... the rendered HTML should be along the lines of ....
<a href="http://www.example.com/some-page.aspx/123/">Some Page</a>