Store your work duration as a total seconds, and post process it to hh:mm:ss for presentation only. It makes the math easier a lot easier than forcing it into a DateTime which is wholly inappropriate.
A DateTime is a "marked point" in time which consists of a number of ticks since a fixed point in the past, not a duration - what you need is a TimeSpan which SQL does not support.
Storing it as seconds lets you sum them easily, and then a simple SQL Function will convert it to a presentation string:
CREATE FUNCTION [dbo].[ConvertSecondsToHHMMSS]
DECLARE @hh INT;
DECLARE @mm INT;
DECLARE @ss INT;
SET @hh = @seconds /60 / 60;
SET @mm = (@seconds / 60) - (@hh * 60 );
SET @ss = @seconds % 60;
RETURN CONVERT(NVARCHAR(9), @hh) + ':' +
RIGHT('00' + CONVERT(NVARCHAR(2), @mm), 2) + ':' +
RIGHT('00' + CONVERT(NVARCHAR(2), @ss), 2)