How to convert number to time format(HH:MM:SS) in ssrs

Hi,

I tried this expression its worked

=Format(DateAdd("s", Fields!MySecondsField.Value, "00:00:00"), "HH:mm:ss")

But my problem is,

I want to calculate the total time,i did like this

got error like #Error

=sum(Format(DateAdd("s", Fields!MySecondsField.Value, "00:00:00"), "HH:mm:ss"))

is it right way?

can any one help me?