(1) epidiatassomai, "to add to," "to arrange in addition": Found only in Ga 3:15, which may thus be paraphrased: "To take a familiar illustration: even a man's will, when ratified, no third party may annul or supplement" (Dummelow, in the place cited.).
⇒See the definition of add in the KJV Dictionary
(2) epitithemi, "to put upon," "If any man shall add unto them, God shall add unto him the plagues" (Re 22:18). The book is not to be falsified by addition or excision (see BOOK) by the interpolation of unauthorized doctrines or the neglect of essential ones (compare De 4:2; 12:32).
M. O. Evans