How to convert negative time span to positive in SQL server

Question

I have a situation in which some time duration is in negative format and stored as string in SQL database as

-06:34:41

and some of will be in positive format as

02:00:00 and

01:00:00

Now I need to add/sum these time duration in select query using SQL Server 2008 such that result will be

-03:34:41

and then after calculation i need it back into string format .Thanks in advance.