SELECT CAST(SUM(CAST(yourDateTimeColumn AS FLOAT)) AS DATETIME) FROM YourTable

CREATE TABLE #tmp(d TIME);
INSERT INTO #tmp VALUES ('14:00:20'),('16:39:53'),('16:39:53')

SELECT DATEDIFF(dd,0,col1) AS Days, CAST(col1 AS TIME) AS HoursMinutesSeconds
FROM
(
	SELECT CAST(SUM(CAST(CAST(d AS DATETIME) AS FLOAT)) AS DATETIME) AS col1
	FROM #tmp
) s

declare @test table
(
timeval time
)


insert @test
values('16:39:53'),
('14:00:20'),
('14:00:20'),
('14:00:20'),
('14:00:20'),
('09:56:29'),
(NULL)

SELECT EffTime/(24*60*60) AS days,
CAST(DATEADD(ss,EffTime%(24*60*60),0) AS time) AS timepart
FROM
(
SELECT SUM(DATEDIFF(ss,0,timeval)) AS EffTime
FROM @test
)t


days	timepart
3	10:37:42.0000000


for more idea on date operations refer

http://visakhm.blogspot.com/2012/07/generate-datetime-values-from-integers.html