This is defined in the cell s attribute which is a reference to the styles.xml.

<row> <c s="1"><v>11</v></c> </row>

You can either find out the right s value in an existing xlsx template you use or use xlsx helpers to update the styles.xml. Like here

https://playground.jsreport.net/w/anon/nOIQZfMt

{{#xlsxAdd "xl/worksheets/sheet1.xml" "worksheet.sheetData[0].row"}} <row> <c s="1"><v>11</v></c> </row> {{/xlsxAdd}} {{#xlsxReplace "xl/styles.xml" "styleSheet.cellXfs"}} <cellXfs count="2"> <xf numFmtId="0" fontId="0" fillId="0" borderId="0" xfId="0"/> <xf numFmtId="2" fontId="0" fillId="0" borderId="0" xfId="0" applyNumberFormat="1"/> </cellXfs> {{/xlsxReplace}} {{{xlsxPrint}}}

As always with xlsx transformations, it requires you to check how the XML structure looks like inside existing xlsx files and replicate the same with xlsx helpers.