Quick and Dirty Workaround
Type your number as normal in (say) cell A1
Input the following formula in B1:
=left($A1,1)
Input the following formula in C1:
=mid($A1,2,1)
Copy the formula across as many columns as you need, increasing the "middle" number each time, so that in D1 you would have =mid($A1,3,1) and so on.
I've ben thinking about your problem and think I have a solution.
Only problem is that it will be hairy.
Your going to have to write an EVENT HANDLER or MACRO that is always running which will PARSE your input.
That doesn't seem to bad; yet, you'll have to test keystrokes for the occurance of any key (ie, ARROW, PG UP/DN) that interrupts the normal progression of data input.
On top of that, you'll have to analyze the CHARACTER SET if you allow your users to select any other keys besides A-Z.
IT CAN BE DONE but it will be a major problemo.
Reminds me of the DOS programming days when the coder had to anticipate every keyboard press...