fill out vs supplement

fill out is not a noun while supplement is a noun.

Word	Noun	Adjective	Verb	Adverb
fill out	No	No	Yes	No
supplement	Yes	No	Yes	No

As verbs, supplement is a hypernym of fill out; that is, supplement is a word with a broader meaning than fill out:

fill out (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

fill out (verb)	supplement (verb)
become round, plump, or shapely	add to the very end
make bigger or better or more complete	add as a supplement to what seems insufficient
write all the required information onto a form	serve as a supplement to
make fat or plump
line or stuff with soft material
supplement what is thought to be deficient