The problem is that a dataport simply generates a TXT file, you simply are using excel to open it as a CSV file. If you export it as a TXT file, then when you open it in EXCEL you will get the Text file wizard which will enable you to define the field properties. The other option is to open it within Notepad.
Andrew is correct - the standard way to export to Excel from Nav is to use the Excel Buffer table (table 370) and functions. For your situation you can set the last argument on the AddColumn function to ‘@’ to format as a string i.e. assigning the numberformat field. Look at the export to excel option in the budget screen for examples.
With dataport there is no technical issue to choose the “format of the cell” (Standard, Text…) coz the standard way is not dataport; it’s Excel Buffer use.
With the Excel Buffer T370, It works : I have tried with NumberFormat field in table 370 coz the code I have changed to test doesn’t use the AddColumn function… (AddColumn function in table 370 uses the field NumberFormat to fix the format of Excel cell)