When I import to excel from our accounting system, negative numbers show the
minus sign at the right e.g. 100- instead of -100 or (100) Excel reads this
as text and I have to go through the list manually inserting a minus sign at
the left and deleting the minus sign from the right.

Is there any easier way to do this? The numbers are not a fixed length, so
the - could be the third or the thirteenth character in the cell.