How can I add namespaces to the root element of my XML using XSLT?

Here's how I'd do it in XSLT 2.0...

XML Input

<Request>
    <Info>
        <Country>US</Country>
        <Part>A</Part>
    </Info>
</Request>

XSLT 2.0

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output indent="yes"/>
    <xsl:strip-space elements="*"/>

    <xsl:template match="@*|node()">
        <xsl:copy>
            <xsl:apply-templates select="@*|node()"/>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="*" priority="1">
        <xsl:element name="{local-name()}" namespace="http://hgkl.kj.com">
            <xsl:namespace name="xsi" select="'http://www.w3.org/2001/XMLSchema-instance'"/>
            <xsl:namespace name="xsd" select="'http://www.w3.org/2001/XMLSchema'"/>
            <xsl:apply-templates select="@*|node()"/>
        </xsl:element>
    </xsl:template>

</xsl:stylesheet>

XML Output

<Request xmlns="http://hgkl.kj.com"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <Info>
      <Country>US</Country>
      <Part>A</Part>
   </Info>
</Request>

Here's an XSLT 1.0 option which produces the same output, but requires you to know the name of the root element...

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output indent="yes"/>
    <xsl:strip-space elements="*"/>

    <xsl:template match="@*|text()|comment()|processing-instruction()">
        <xsl:copy>
            <xsl:apply-templates select="@*|node()"/>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="/Request">
        <Request xmlns="http://hgkl.kj.com" 
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
            xmlns:xsd="http://www.w3.org/2001/XMLSchema">
            <xsl:apply-templates select="@*|node()"/>           
        </Request>
    </xsl:template>

    <xsl:template match="*">
        <xsl:element name="{local-name()}" namespace="http://hgkl.kj.com">
            <xsl:apply-templates select="@*|node()"/>
        </xsl:element>
    </xsl:template>

</xsl:stylesheet>