à la piano

If you could map the key velocity to the volume slider, it might work.  I didn't see an option for velocity to route directly that way in Organteq when I checked, but it might be buried in a menu I didn't look at.  It might also be possible in a DAW where MIDI messages can be converted (like velocity to a CC); I believe Cubase has that kind of filtration in most higher-end versions of it (at least in Artist and Pro), which would allow you to play in realtime that way.  I don't know about other DAWs/Hosts, but I would guess that most higher-end ones would have that kind of programming built-in somehow.

While it isn't very "organ-like" in how organs are built and played today, it is actually very common in Baroque and older organs for there to be an element of "touch" which--while not necessarily changing the volume--changes the timbre of the sound by how quickly or slowly the pipes receive air.  While the effect can be badly executed, it usually adds a warmth, spontaneity, and authenticity to the sound.  After 19th Century improvements that made the wind-chest more consistently pressurized and the key action more consistent (which had to be pressurized and counter-balanced because larger pipe organs meant more internal air pressure and more effort needed to press a key to open pipes which got to the point that full organ meant pipes wouldn't open at all), those subtleties of organ-playing largely went away, but particularly for historic instrument recreation and Baroque organs, having a variable touch and timbre would be very accurate and desirable to add to Organteq.

I hope Organteq does consider it. One of my favorite features of pianoteq is the ability to create completely new instruments that wouldn't be possible or practical in the real world. For example, not only can you add a sustain pedal to a harpsichord and play at different volumes depending on key velocity, but you can also have the timbre change dramatically with the dynamic change, so a gentle velocity produces a very warm tone, like a fleshy fingertip, while a forceful velocity produces a very hard tone filled with ringing overtones. Similar abilities with an organ model would be fantastic.

I'm learning to play the organ now. This is unbelievable!

my idea was to have a function that allows the keyboard to react to different velocities, as is the case for pianos, moreover, this possibility exists on the harpsichords offered by Modartt, an option that would allow, perhaps , a different and innovative use of the expressive possibilities, but as I am totally ignorant on the subject (organ and playing techniques) I may say nonsense

It's true that it would be great, I've tried several piano synthesizing programs, and I'd LOVE to have one with the same amount of effort put in it as OrganTeq. I mean, that's one of the main reasons why I've more or less stopped working with piano synthesizers and am now only working with organ sounds, but the organ can not necessarily convey all of the emotions and sonorities that the piano conveys and that I need in my project (the composition of the music for a narrative game).