(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), (c), 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.