bolt-on

/B OW1 L T AA0 N/
noun
  1. 1

    An accessory that can be bolted on or otherwise attached.

  2. 2

    A functionality or service that can be added to an existing arrangement or plan (for example, a mobile telephone subscription).

adjective
  1. 1

    Of an accessory: capable of being bolted on or otherwise attached.

  2. 2

    Used to extend an existing system, operating as an add-on or plug-in.

Source: https://en.wiktionary.org/wiki/bolt-on