I believe you would need to make morphs in an outside application.  You would export each of the ten body parts separately, from the zeroed pose, and shorten the nails there.  Each of the resulting OBJs would need to be imported as MTs on the properties tabs of the affected parts.

With nothing else morphed, you could then make a full body morph which would give you a single dial to shorten all nails at once.  Save that figure back to the library with a unique name.

Hope this makes sense.  I just got up ;)

John