I believe it is just C = [A B]
This creates C as a one-row matrix, with the entries of B appended behind the entries of A.
Doesn't that create a matrix with two rows of 7 columns each?
I read the OP's question as just wanting to append four 9's to the 1-row array, but leave it a single row array.
(Hmmnn... maybe I misunderstood the OP's request. Although, if we read the question literally, he also wants the first three outputs of the second row to be dashes.)