(aa), (bb), etc. as numbering format?

Is it possible to set automatic row numbering to use the format (aa),
(bb), (cc), etc.? Setting (a), (b), ©, etc. is no problem, but I
can’t seem to figure out how to get the double-letter format (which
is, by the way, used by some federal statutes, and it would thus be
helpful to be able to reproduce in OmniOutliner). Thank you.

– copied the question fromt he old blog, as I have the sam eissue/problem
(http://forums.omnigroup.com/showthread.php?t=3065)