usurp vs annex

usurp is not a noun while annex is a noun.

Word	Noun	Adjective	Verb	Adverb
usurp	No	No	Yes	No
annex	Yes	No	Yes	No

As verbs, annex is a hyponym of usurp; that is, annex is a word with a more specific, narrower meaning than usurp:

usurp: seize and take control without authority and possibly with force; take as one's right or possession
annex: take (territory) as if by conquest

Other hyponyms of usurp include appropriate, capture, conquer, seize, preoccupy, hijack, raid.

usurp (noun)	annex (noun)
	an addition that extends a main building

usurp (verb)	annex (verb)
seize and take control without authority and possibly with force; take as one's right or possession	add an extra part to
take the place of	take (territory) as if by conquest