How can I format a sum field on a data report?

I have created a data report and one of the problems is how to format a sum field in the way I want? For example: I have a column named BillingUnits which includes duration in seconds and another one name CDuration which includes thesame duration in a string format like this "00:03:45" (hh:mm:ss). This specific field, in the first column has this value: "225" which equals exactly the same duration but it is in seconds. What I want is to display the sum on the report in the format (hh:mm:ss). I have two solutions to do it: First, to sum the fields of column with seconds and format the result. But how can I format a function on a data report. Second solution is to sum the column which has the right format, but I think I cannot add strings as time format? Any other solution???
:-(

Michael Vlastos
Automation Engineer
Company SouthGate Hellas SA
Development Department
Athens, Greece