Hi Pankaj2, 

Thank you for reaching out on this. 

Unfortunately, each export to Excel from QBO would create a new Excel file. 

May I ask what is your intention of exporting to an existing Excel file and how would you like it to be handled? Would you like the newly exported report to be in a tab in the existing Excel file, or...?

Would like to find out more and see if we can help you with this. Thank you.