Rgr that Einstein,
In the game options, click on Controls, go to Modifiers cell bottom left this will bring up the modifier panel and shows a list of available modifiers, ie, Alt, Ctrl, Mouse etc etc
Click the ADD button which will bring up another panel with Device name which has a drop down triangle, default is Keyboard, once you click the drop down your stick device will show, click on it.
Under that cell is another, Select Modifier button again with a drop down triangle, click on the drop down and you will see a list of every button you have on your stick, scroll down to relevant button, ( Mine was Joy btn 7 ) click on it and it's done.
I don't know if this will work for all types of stick , but worth a try.
If you a profile alrready loaded and some of your cells were red'ed out , these should now disappear and you'll see your modifier button.
For instance Joy button 21 on my throttle is Coolie Hat Up, with my pinkie ie btn 7 , it becomes Boat Switch Fwd.
Hopefully that's clear as mud
Happy flying
keep low move fast