Does the following help?

declare @ TimeVal time,

@ TotalTime time; -- spaces between @ and rest of variable names to allow posting of code

set @TimeVal = '08:05:44.0000000';

set @TotalTime = '00:13:00.0000000'

select @TimeVal, @TotalTime, dateadd(ss, ((DATEPART(HOUR,@TotalTime) * 60) + DATEPART(MINUTE,@TotalTime) * 60) + DATEPART(SECOND,@TotalTime), @TimeVal);