I'm not sure I get the question...

The following worksheet.sheetData[0].row[2].c[1] should represent the 3th row and 2nd column inside the xslx.
This doesn't necessarily mean its cell C2, because the previous cells can be empty and in this case excel doesn't add them to the row collection. You can unzip the excel template, find file sheet1.xml and find whats inside the sheetData.row nodes. Or just try to run the template in jsreport, replace cell and see where it gets actually replaced.