Try the following, adapted from
http://www.xsltfunctions.com/xsl/fun...o-length.html:
<xsl:function name="f:pad-string-to-length" as="xs:string"
xmlns:f="....." >
<xsl:param name="stringToPad" as="xs:string?"/>
<xsl:param name="padChar" as="xs:string"/>
<xsl:param name="length" as="xs:integer"/>
<xsl:sequence select="
string-join (
($stringToPad, for $i in (1 to ($length - string-length($stringToPad)) return $padChar)
,'')
"/>
</xsl:function>