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Synonyms for Supplement (same or very similar meaning)

WordNet sense 1 (textual matter that is added onto a publication; usually at the end):
addendum, postscript

WordNet sense 2 (a quantity added (e.g. to make up for a deficiency)):
supplementation

WordNet sense 3 (a supplementary component that improves capability):
accessory, add-on, appurtenance

WordNet sense 4 (add to the very end):
affix, append, affix, add on


From the ODE community, based on WordNetadd/edit