add-on

lisälaite, lisä Something which can be appended to something else.
puhekieltä A software extension or hardware peripheral that provides additional functions or customization for a core application or system.
2010, Kenneth Baldauf, Ralph Stair, Succeeding with Technology (page 77)

Microdrives that can hold several gigabytes (see Figure 2.20) have been available as add-ons for portable devices.