add on vs supplement

add on is not a noun while supplement is a noun.

Word	Noun	Adjective	Verb	Adverb
add on	No	No	Yes	No
supplement	Yes	No	Yes	No

As verbs, add on and supplement are synonyms defined as:

Other synonyms of add on include affix, append.

add on (noun)	supplement (noun)
	a supplementary component that improves capability
	a quantity added (e.g. to make up for a deficiency)
	textual matter that is added onto a publication; usually at the end

add on (verb)	supplement (verb)
make an addition	add to the very end
add to the very end	add as a supplement to what seems insufficient
	serve as a supplement to