Declare @S varchar(50)='2017092018'

Select convert(datetime,left(@S,8)) + convert(datetime,right(@S,2)+':00')

2017-09-20 18:00:00.000

DECLARE @d NVARCHAR(10)='2017092018';

SELECT DATETIMEFROMPARTS(LEFT(@d, 4),SUBSTRING(@d,5,2),SUBSTRING(@d,7,2), RIGHT(@d,2),0,0,0 ) ;