The Collaborative International Dictionary of English v.0.48:

Unconvertible \Unconvertible\
   See convertible.

WordNet (r) 3.0 (2006):

unconvertible
    adj 1: used especially of currencies; incapable of being
           exchanged for or replaced by another currency of equal
           value [syn: inconvertible, unconvertible,
           unexchangeable] [ant: convertible, exchangeable]