Apparently not. But that's no particular limitation, as you can easily create an additional custom slide show which consists merely of the same slides as used in the original custom shows - in the appropriate order.
You can do this slightly more easily than creating the entire new list of slides from scratch:
o In the Custom Slide Shows dialogue, select one of the existing custom shows - probably, sensibly, the longest.
o Click Copy.
o Click Edit... .
o Rename the new custom show and add the other necessary slides in the appropriate places.
I trust this helps.
Brian Barker