<msxsl:script implements-prefix="str" language="Javascript">
    <![CDATA[
    function padding(num, str){
            var opString = "";
            var pad = " ";
            if (str != "" && str != null) pad = str;
            for (; opString.length < num;){
                    opString += pad;
            }
            return opString.substr( 0 , num);
    }
    ]]>
  </msxsl:script>

<xsl:value-of select="str:padding(Number,'8')" />