DECLARE @d varchar(6) = '20114'

SELECT try_convert(date, substring(@d, 1, 4) +

CASE WHEN len(@d) = 5 THEN '0' ELSE '' END +

substring(@d, 5, 2) + '01')

I assume that 201110 would be 2011-10-01

I added try_convert, since most likely you have values in that column that are illegal.