<NewDataSet>
<Main>
<Marka>Q1</Marka>
<Kol> 1 </Kol>
</Main>
<Main>
<Marka>Q1</Marka>
<Kol> 3 </Kol>
</Main>
<Main>
<Marka>Итого по марке</Marka>
<Kol> 4 </Kol>
</Main>
<Main>
<Marka>Q2</Marka>
<Kol> 5 </Kol>
</Main>
<Main>
<Marka>Итого по марке</Marka>
<Kol> 5 </Kol>
</Main>
</NewDataSet>

<xsl:stylesheet id="my_xslt" version="1.0" 
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
  xmlns="urn:schemas-microsoft-com:office:spreadsheet" 
  xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet">
  <xsl:output method="xml" encoding="windows-1251"/>

  <xsl:key name="Марка" match="/NewDataSet/Main/Marka/text()" use="." />
  <xsl:template match="/">
....
  <xsl:for-each select="/NewDataSet/Main/Marka/text()[generate-id()=generate-id(key('Марка',.)[1])]">
    <Row>
      <Cell ss:StyleID="s29">
        <Data ss:Type="String">
          <xsl:number format="1" value="position()"/>
        </Data>
      </Cell>
      <Cell ss:StyleID="s30">
        <Data ss:Type="String">
                   <xsl:value-of select="."/>
        </Data>
      </Cell>
      <Cell ss:StyleID="s31"/>
      <Cell ss:StyleID="s32"/>
      <Cell ss:StyleID="s33"/>
      <Cell ss:StyleID="s32"/>
    </Row>
  </xsl:for-each>