temp = sum(A.*(10.^(length(A)-1:-1:0)))
B(2) = temp;
This is under the assumption that you want to take the digits of the individual array elements and use them to create a single decimal number to be stored.
If instead you want the second element of B to become the vector of values so that when you index B(2) you get back the vector as distinct elements, then that cannot be done with a numeric array, and you would have to use a cell array instead.