It may be as simple as ensuring correct data type is specified.
For example I use the "Excel Report" function panel (..samples\activex\excel\excelreport.fp) and this line:
ExcelRpt_SetCellValue (ws_handle, cell, ExRConst_dataString, outstring);
So, having read the text box string into your character array "outstring", the above line should work. Note you need "ExRConst_dataString", not "ExRConst_dataDouble".
Hope this helps.
--Ian