Stephen, "next asInteger" transforms the character into its ASCII value (e.g. space = 32, A=65, etc.)
To remember the previous, you could add a separate variable (e.g. 'prevnbr') and store 'nbr' into this var at the end of the loop (e.g. prevnbr := nbr.), so you would have both current and previous values available to you as you set up the arrays.
As to combining previous with current characters into a String, I do not have a direct answer. Do you want these as integers instead of characters? If you want as ints, you can just add 'prevnbr' and 'nbr' together. I've attached a quick example using the code above that just displays the current and previous values and adds them together. these display in a debug window so you can see.