The Collaborative International Dictionary of English v.0.48:

Supplementation \Sup`ple*men*ta"tion\, n.
   The act of supplementing. --C. Kingsley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supplementation
    n 1: a quantity added (e.g. to make up for a deficiency) [syn:
         supplement, supplementation]
    2: the act of supplementing [syn: supplementation,
       subjunction, subjoining]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "supplementation":
   accession, addition, adjunct, adjunction, affixation,
   agglutination, annexation, attachment, augmentation, increase,
   joining, junction, juxtaposition, prefixation, reinforcement,
   suffixation, superaddition, superfetation, superjunction,
   superposition, uniting