The function module that you are calling downloads the ITAB as a .txt file, then opens excel and presses the SaveAS button to convert this to Excell.
My suggestion would be to modify the function module to open excel, and 'open' your .txt files per tab broken down into as many files as you need.
1) Download the data into no of .txt files.
2) Open Excel
3) On the first 'TAB' select open File and specify the file name
4) Select the second tab and repeat point 3
5) Select SaveAs to convert to .xls
A bit of OLE type coding involved so its a case of 'do they really need this'?