The Collaborative International Dictionary of English v.0.48:

Superable \Su"per*a*ble\, a. [L. superabilis, from superare to
   go over, to surmount, fr. super above, over.]
   Capable of being overcome or conquered; surmountable.
   [1913 Webster]

         Antipathies are generally superable by a single effort.
                                                  --Johnson.
   [1913 Webster] -- Su"per*a*ble*ness, n. -- Su"per*a*bly,
   adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

superable
    adj 1: capable of being surmounted or excelled [syn:
           conquerable, superable]