Can we add a finding to a paper which is known by other communities but not ours? I’ve been working on a research project, and we have some findings that we are now writing a paper. However, I am unsure about one section of the paper. 

We have done some experiments (related to images and videos), and after quite a lot of reverse engineering, we discovered something related to how cameras take pictures and videos. Later, we read a lot about camera internals and found out our finding is already there. So, there is nothing novel about it.

However, previous research/other researchers in my field don’t know it and due to that the performances of similar experiments were around 70–80 % whereas ours is 96–97 %. I wonder if our discovery is a valid to include to a paper. Or what is a good way to add it to the paper and make it look fancier than it is?
Taking an approach to one problem and translating it to another problem is a valid scientific contribution and can be a considerable creative achievement.
I know a handful of important papers that essentially do this.
If you actually achieved an improvement and it was a very obvious thing to do, why didn’t people do it before?

As you have already noted, the crucial point is how you sell your insight.
The first step is to identify the level on which you actually solved an unsolved problem (in a broad sense that captures performance boosts).
For example, you may not have developed a new method to track the movement of fast-moving objects, but you have found a new approach to study the movement of arrows by translating established techniques for studying the movement of sparrows.
This should be the leitmotif of your paper (or the respective part thereof).
Also, do not forget to cite previous studies which could have been improved by your insight.

The only thing that probably nobody cares about is your reverse engineering (unless you developed some new reverse-engineering methodologies or similar).
But as a scientist you are in company when spending a considerable amount of time on something that you could have achieved easier.
Science always looks simpler when it’s finished and nicely wrapped.