I have an existing Excel worksheet and I want to be able to add new data to
cells in that sheet. Basically the worksheet contains expense information.
As new receipts are submitted, I want to be able to add these numbers to the
relevant cells, I would like it if all I had to do was type in the new number
and Excel assumed I wanted that new number added to the previous value of the
cell.

Such that.
Existing value for cell (let's say) A1 is $45.00

and I have a new expense of $15.00

all I would do as a user is select cell A1 and key in 15 and then hit
<enter or <tab and this new value is added to 45 that is already there and
the value for cell A1 is now $60.00

Thanks
Chris