supplement vs add on

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

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

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

Other synonyms of supplement include affix, append.

supplement (noun)	add on (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

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