A word which describes an external add-on that provides new essential functionality

As the title says, I'm looking for a word to describe some clip-on module which provides a new ability that improves the base concept/entity.

Some context, such a word could describe:

• A warp drive module on a space ship
• A standalone plugin which provides a completely new feature set

If possible, the word might imply a necessity, that without such an add-on the entity is lacking and incomplete.

Solution 1:

I'd suggest add-on:

• a device or unit added to equipment or a construction.
• a hardware device, such as an expansion card, that is added to a computer to increase its capabilities.

The Free Dictionary

Solution 2:

First thought was optional upgrade or enhancement:

An increase or improvement in quality, value, or extent.