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Synonyms for Accession (same or very similar meaning)

WordNet sense 1 (something added to what you already have):

WordNet sense 2 (agreeing with or consenting to (often unwillingly)):

WordNet sense 3 (the right to enter):
access, admittance, entree, entrée

WordNet sense 4 (the act of attaining or gaining access to a new office or right or position (especially the throne)):
rise to power

From the ODE community, based on WordNetadd/edit