Etymology of addendum:
1794, literally “something added,” from Latin addendum
Etymology of add:
Late 14c., “to join or unite (something to something else),” from Latin addere “add to, join, attach, place upon,” from ad- “to” + -dere comb. form meaning “to put, place,” from dare “to give”. Meaning “to do sums, do addition” also is from late 14c. Related: Added; adding. To add up “make sense” is from 1942.