The Collaborative International Dictionary of English v.0.48:

Pregnable \Preg"na*ble\, a., [F. prenable. See Impregnable.]
   Capable of being entered, taken, or captured; expugnable; as,
   a pregnable fort. [R.] --Cotgrave.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "pregnable":
   aidless, assailable, attackable, beatable, conquerable,
   defenseless, expugnable, fatherless, friendless, guideless,
   helpless, leaderless, motherless, penetrable, surmountable,
   unfriended, unprotected, untenable, vincible, vulnerable, weak