Right. Assuming you are using any SF2 soundfont synthesizer, the soundfont must contain a "modulator" to capture "polyphonic (key) aftertouch pressure" MIDI event and route this event to a given "generator". If this kind of "modulator-to-generator" doesn't exist, the synthesizer does nothing when it receives this "key aftertouch pressure" MIDI event. You can add any "modulator(s)" yourself by editing the soundfont. If you are unfamiliar to Soundfont SF2 concepts please read SFAPP21.PDF then sfspec24.pdf.
I see, thanks. Worst case scenario, I can try it myself so I can test it, which I guess will be what it takes, cause I don't bet I'll find a font with that :)
Now, I'm just landing here, this is my project https://github.com/porres/pd-fluidsynth
by the way, a Pd object that loads fluidsynth. Forgive me if I'm too newbie, but I'd also like to know more about "generators" since you mentioned it. With my object, I can send "generator" messages directly to fluidsynth and I'm wondering how to test them. I see gen 17 is pan, for example, but it didn't seem to work for me. What is the input range for those, 0-127 as well? Does the soundfont need to have this enabled somehow? I should also edit the soundfont to test those?