The Collaborative International Dictionary of English v.0.48:

Dupable \Dup"a*ble\, a.
   Capable of being duped.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "dupable":
   cullible, deceivable, deludable, easy, exploitable, foolable,
   green, gullible, hoaxable, hoodwinkable, humbugable, ingenuous,
   naive, persuadable, seduceable, simple, soft, unsophisticated,
   victimizable