Yes, you can use the usual design pattern of a modified identity transform:
<xsl:template match="*">
<xsl:copy>
<xsl:copy-of select="@*"/>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>
<xsl:template match="/*">
<xsl:copy-of select="*"/>
<time><xsl:value-of select="$timestamp"/></time>
</xsl:template>
If you want <time> to be in the same namespace as its parent element, create it using <xsl:element name="time" namespace="{namespace-uri(.)}"/>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference