An addition, an extension.

An appendix to a book or document.

An addition or extension to a building.

An addition to the territory of a country or state, from a neighbouring country or state, normally by military force.

To add something to another thing, especially territory; to incorporate.

"The ancient city of Petra was annexed by Rome."

To attach or connect, as a consequence, condition, etc.

"to annex a penalty to a prohibition, or punishment to guilt"

To join; to be united.