This starts getting difficult once you have empty cells in the spreadsheet, but for the moment it appears to be fully populated, in which case you can do
Code:
<xsl:template match="Table">
<xsl:variable name="table" select="."/>
<xsl:for-each select="Row">
<xsl:variable name="row" select="."/>
<char>
<xsl:for-each select="1 to count($table/Row[1]/Cell)">
<xsl:variable name="col" select="."/>
<xsl:attribute name="$table/Row[1]/Cell[$col]/*:Data"
select=$row/Cell[$col]/*:Data"/>
</xsl:for-each>
</char>
</xsl:for-each>
</xsl:template>