You can't do this with Word's inbuilt numbering scheme, but you could use a set of SEQ fields like:
{QUOTE{SET A2Z{=MOD({SEQ ABC}-1,26)+1}}{SET AA2ZZ{=INT(({SEQ ABC \c}-1)/26)}} {IF{AA2ZZ \* ALPHABETIC}= " " "" {AA2ZZ \* ALPHABETIC}}{A2Z \* ALPHABETIC}}
This will produce the sequence: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z, AA, AB, AC, AD, AE, AF, AG, AH, AI, AJ
Note: The field brace pairs (ie '{ }') for the above example are created in the body of the document via Ctrl-F9 (Cmd-F9 on a Mac) - you can't simply type them or copy & paste them from this message.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]