Like this?

declare @ TimeVal time,

@ TotalTime time; -- space added between @ and variable name to allow code to post

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),

@TimeVal + @TotalTime;

I get the following error:

Msg 8117, Level 16, State 1, Line 6

Operand data type time is invalid for add operator.