Despite your statement, A is not a cell array, but a 2D char array. You can convert A into a cell array of char vectors using cellstr. So:
C = [cellstr(A), num2str(B)]
would be the simplest way to obtain what you want.
Note: Since R2016b, string is a new type different from the char arrays you're using. To avoid ambiguity use the term char array (or vector) instead of string.
