The case of outnumbered numerator

Hello Pundits

We have a six digits numerator that approaches 899999.

Since it happens to be a CHAR field I would like to start using the
letters A-Z
in lexical order when incrementing this field so that incrementing
899999 will result in 9AAAAA , 9AAAAB , ......., 9AAAAZ, 9AAAA0, 9AAAA1 etc.
(This way the numerator will sort the EBCDIC natural way)

Do you know an *elegant* way of doing it ?
(EBCDIC table having "holes" of unprintable chars does not expedite matters)

Thanks
Gad