Well, what I eventually did : I opened the X-Arcade, removed the PCB (!!) and hooked the db9 cable wires to the arcade buttons by using a bonding lug... This is super straightforward and extremely simple but, well, then, it only works with the Amiga.... :-/ And you kinda destroy your stick !!
OR you can only wire some buttons and keep the others connected to the X-A PCB : this way, you can map keyboard keys to some buttons (for flippers for instance, spacebar etc.) and the others to the Amiga db9 joystick. [provided you have a PS/2 or USB keyboard adapter on your Amiga / Minimig / MiST...]
I can tell you the experience of playing Amiga games on an arcade stick (along with some additional buttons mapped to the keyboard) is just AMAZING.
Also you can use a button for the "up" direction which also makes many games easier...