LaTeX is a typesetting system, and trying to use it for anything other than that will probably lead you to frustration at some point or another. Unless your table is really very simple, I think going for a spreadsheet and then exporting that to LaTeX is definitely the best way to go.
Now, having said that, for a simple table you can use, as Thorsten suggested, a package like spreadtab and write something like:
If you have an argument as an character, i.e. #1=A, then `#1 will give you the ASCII number of this character. If you want that the character '0' gives you a numeric value of 0, and so on, you simply have to subtract the value `0 from each value. Luckily then characters for the digits are coded in numeric order in ASCII, i.e. `0-`1 = 1 etc.
I would loop over the input text yourself by putting it in front of an end-marker and reading one character a time in a recursive fashion.
% Reads the input one token a time, should only contains normal characters!
\ifx\@nnil#1\relax\else % stop looping when endmarker is read
\expandafter\@checksum % Recursive call => loop
This stores the checksum digit into \checksumdigit and prints it in the text. I tested it successfully on the three books above.