Addendums rhymes with:
sedums, adam's, adams, adams', addams, condoms, fiefdoms, freedom's, freedoms, kingdom's, kingdoms, mcadams, memorandums, modems, needham's, odoms, oldham's, referendums, saddam's, sedums, sheikdoms, tandem's, wadhams
Addendums sounds like:
admittance, admitting, admonitions, attendance, attending, attentions, automating