The Collaborative International Dictionary of English v.0.48:

Supposable \Sup*pos"a*ble\, a.
   Capable of being supposed, or imagined to exist; as, that is
   not a supposable case. -- Sup*pos"a*ble*ness, n. --
   Sup*pos"a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supposable
    adj 1: capable of being inferred on slight grounds [syn:
           presumable, supposable, surmisable]