You could do this not so much with a button, but with a new report parameter...

You could create a string type parameter that allows the user to chose the format mperhaps called Format, before they execute the report. Set the label to something the users understand, Set the value to 1 and 0 or some other.

Then in the format statement for the field use an IIF expression based on the value of the parameter, something like the following pseudocode =IIF(Parameters!Format.value = 0, "HH:MM:SS", "Just Seconds")

-Luke.