If you import your CSV into Excel, you can quickly combine deposit and withdrawal columns using a formula like =IF(D2=0,E2,0-D2). You need to adjust the formula depending on which columns you have withdrawal and deposit data, but once you get the formula correct, it's very simple to move your mouse to the lower right corner of the cell that you typed the formula into and double click and Excel will run down the sheet and put the correct value in every cell. Then you can re-save the file as a CSV and it will import correctly