To take a random sample from a vector, just use randi - no need for randsample. Example:
numbers = [2, 3, 9, 7];
randomNumber = numbers(randi(length(numbers)));
To add a digit to the end of another number, assuming the digit is represented by an integer value in the range 0 to 9, just multiply by 10 and add. Example:
x = 269;
x = 10*x + randomNumber;
... and repeat as necessary.
You can also convert x and the extra digit to strings and concatenate them. That's a little more fiddly, but depending on what you really want to end up with might be a useful technique.
By the way, you say you want to add a digit to the number, but your set of "digits" includes 12. Is that a mistake, or do you actually want to do something more general than adding a single digit to the decimal representation?