The Collaborative International Dictionary of English v.0.48:

Extendible \Ex*tend"i*ble\, a.
   1. Capable of being extended, susceptible of being stretched,
      extended, enlarged, widened, or expanded.
      [1913 Webster]

   2. (Law) Liable to be taken by a writ of extent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

extendible
    adj 1: capable of being lengthened [syn: extendible,
           extendable]