add-on

Short definition from Wiktionary

Noun

1. 1
   Something which can be appended to something else.
2. 2
   A software extension or hardware peripheral that provides additional functions or customization for a core application or system.
3. 3
   Additional chips that can be purchased at specific times in tournament play.
4. 4
   Synonym of expansion pack.
5. 5
   Alternative form of add-on.

Verb

1. 1
   To incorporate (something extra, often an amount of money) into something else (often a bill).
2. 2
   To put an expansion (addition) onto a building, usually a home.