The Collaborative International Dictionary of English v.0.48:

Apposable \Ap*pos"a*ble\, a. (Anat.)
   Capable of being apposed, or applied one to another, as the
   thumb to the fingers of the hand; able to be brought into
   direct spatial opposition.

   Syn: opposable
        [Webster 1913 Suppl. + WordNet 1.5]

WordNet (r) 3.0 (2006):

apposable
    adj 1: capable of being placed opposite to something; "the thumb
           is opposable to the forefinger" [syn: opposable,
           apposable] [ant: unopposable]