I'm looking for a specific word that means to add something which isn't necessary

How about embellishment?

M-W:

embellish: to make beautiful with ornamentation; to heighten the attractiveness of by adding decorative or fanciful details

embellishment: something serving to embellish; ornament

Embellishments are often considered unnecessary or inessential to functioning, like the hood ornament on a Rolls Royce.

Edit: Another possibility is extraneous.

From M-W:

extraneous: not forming an essential or vital part; having no relevance

Presumably, junk DNA is extraneous (although no one added it; it just worked out that way over time).

A terrific, highfalutin word for that is

supererogatory adj
1. going beyond the requirements of duty.
2. greater than that required or needed; superfluous.

TFD Online

You will usually hear this said in a derogatory (same root) manner, implying that something is pretentious, done only to impress without possessing any real utility or merit.

You have rejected 'redundant' because you say it implies backup in case of failure. It can mean that, particularly in systems design as in the provision of redundant capacity for a circuit, duplexing a server or using hard drives in parallel (the acronym RAID stands for Redundant Array of Independent Disks after all), but it can also mean unnecessary or no longer necessary.

The most familiar sense of "redundancy" to many people is that of people becoming unemployed because their job is no longer needed due to cutbacks, automation, the closure of a place of work (sometimes because production has been moved to another country) or the failure of the employing company. In this case making a person "redundant" means that that person's contribution is no longer necessary to the employing organisation.

You mention 'junk' DNA which is so called because, as it does not code for proteins, it was once thought to have no function (geneticists are identifying functions for much junk DNA now) but even under the old view of junk DNA there was a theory that it once had a function that was no longer being carried out, therefore it was redundant.

It is also possible to add redundant functionality to a product. An example would be the inclusion of a circuit and screen which does nothing but display a complicated pattern of lights at power up in a purely practical item like a refrigerator when a simple LED would do. The inclusion of washing machines in the 'internet of things' seems like redundancy to many people.

There may be those who consider that there is redundancy in this answer since I seem to have gone on at length, but these wider definitions of "redundancy" seem to fit your requirements perfectly.

Superinduce:

to introduce as an addition over or above something already existing.

Reference:
http://merriam-webster.com/dictionary/superinduce