I thank you very much for all of your answers, opinions and informations.
First of all sorry for my English, I'll do my best.
Before to enter in details into my needs, in my first post I've aksed if exist some companies on the market which offer scripting services for NVDA. Of course paying for that.
In the next lines I'll try to explain as short and clear as possible the situation. Any kind of suggestion to solve it from people with more experience than me, will be well accepted. I thank you in advance for the patience.
I am a stenographer. I use a steno machine, which is a keyboard, to transcribe registered audio from Court. For those who can see, here's a short video of me, typing on the keyboard:
On a standard QWERTY keyboard you type letter by letter, while on a steno keyboard you type syllable by syllable, or word by word.
It is much faster. If you are a good stenographer you can type in realtime what a speaker says. Stenographers are also used for realtime subtitling in television.
In Italy I didn't find any other visually impaired person who makes this job. I don't know if it is a specific problem of Italy, but here most of the blind and visually impaired people's job is yet telephone receptionist.
For this reason the Italian National Organisation for Blind and Visually Impaired People I belong to, is interested to share and diffuse this kind of job for those who are interested.
I began to lose seriously my sight on January 2015. So I've left my job and begin to study stenography.
On January 2017 a company hires me and since that moment I'm doing this job. But since three years I'm building and improving a workflow, to make it 100% accessible.
At the moment I can say it is 90% accessible. I miss the last step, and it is the reason of this post and the need of a custom add-on.
Until this job is not 100% accessible me and the Organization I belong, we cannot share and diffuse.
If you run the youtube video above, you can hear the human speaker, and also the NVDA vocalizer voice which repeats what the speaker says, while I'm typing.
The voice helps me to know if what I'm typing is right or not. Of course I should concentrate on two voices at the same time. It is like to listen continuously an echo. After a bit of practice the brain can get used to it.
Basically the main problem is in case I make a mistake. In that situation I press a button on the keyboard which works exactly like an "undo", or a CTRL+Z: it deletes syllable by syllable or word by word.
In this case NVDA is not able to read. So I have audio feedback when I go forward, while typing, but I have not audio feedback when I go backward, while deleting.
In this video I've explained more in detail the issue, maybe it is too long, sorry for that:
I've tried different Screen Reader, but NVDA is the only one which gives me audio feedback when I type with a steno machine. JAWS doesn't work. JAWS gives me audio feedback only if I type with a standard QWERTY keyboard.
For this reason this is the first community I ask help.
I've red above NVDA is developed in python language. Also the application I use to convert steno keyboard strokes into words and letters on the screen, is developed in python, and it is open source too.
I thank you very much for reading until the end. If you know any company, or studio, or association, od individual person who can help me to solve this problem, I will appreciate a lot, also JAWS if you think is the best solution.