You can't use SUM on a DateTime and varchar column.
If your final goal is to get sum of all datadiff records in hh:mm:ss then stored the difference in minutes as:-
SELECT DATEDIFF(second,CAST('02:32:00' AS DATETIME)
, CAST('03:40:00' AS DATETIME))/60.0 AS MIN_DIFF;
Then to do sum of all minutes in hh:mm:ss then use:-
SELECT CONVERT(CHAR(8), DATEADD(Minute,convert(int, 68), '00:00:00'), 108)