The Collaborative International Dictionary of English v.0.48:

Modifiable \Mod"i*fi`a*ble\, a. [From Modify.]
   Capable of being modified; liable to modification.
   [1913 Webster]

WordNet (r) 3.0 (2006):

modifiable
    adj 1: capable of being modified in form or character or
           strength (especially by making less extreme); "the rhythm
           of physiological time is not modifiable except by
           interference with certain fundamental processes" - Alexis
           Carrel [ant: unmodifiable]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "modifiable":
   able to adapt, adaptable, adjustable, alterable, alterative,
   changeable, checkered, convertible, ever-changing, flexible, fluid,
   impermanent, kaleidoscopic, malleable, many-sided, metamorphic,
   mobile, movable, mutable, nonuniform, permutable, plastic, protean,
   proteiform, resilient, resolvable, rubbery, supple, transformable,
   transient, transitional, transitory, transmutable, variable