DateDiff Result Set to HH:MM:SS in SSRS

I want a value returned from a DateDiff to format to HH:MM:SS and equally do a total sum using  "=TimeSpan.FromTicks"

Currently, I am using:

=DateDiff("S",Fields!StartDate.Value,Fields!EndDate.Value).

How do I modify the above expression  and get the summation in the same format?

Thank you

Zionlite