Check this
create table timed
(hou time)
insert into timed values('9:25:03')
insert into timed values('11:25:33')
select cast(sum(datediff(second,0,hou))/3600 as varchar(12)) + ':' +
right('0' + cast(sum(datediff(second,0,hou))/60%60 as varchar(2)),2) +
':' + right('0' + cast(sum(datediff(second,0,hou))%60 as varchar(2)),2)
from timed