I believe it is possible - though I have never attempted it.
Since excel files (.xlsx) are XML, which is text, you would need to engage in significant substitution() scripting - made worse if you had a variable number of records that needed to be exported.
Manipulating Excel's internal XML is always going to be a very difficult task. You might have to change more then one internal file inside of the Excel file. Also, don't forget, that you need to unzip the Excel file and zip it back when you finished. Any mistakes will result in a corrupted unusable file.
Two other options are - try to use Excel ODBC or try VBScript to add data. Both are going to be easier than changing XML.
Th third option is to import Excel file into FileMaker, add new records and export it with the same file name, overwriting the original. Use a separate table for import/export with the field names matching column names.
.xslx = compressed and not TEXT XML.
for existing Excel, I have done and recommend a worksheet into which the data is imported (from CSV?) and the ability of Excel to use it's functions to make that data appear on other worksheets. That way you can create templates that get the "dynamic" date without having to recreate them again.