I know this thread is old, but I was asking myself the exact same question as the OP (i also have a HOTAS.X and use the button 8 as a modifier) so I decided to post my answer I case someone else has the same problem and, like me, doesn't want to install any program. The solution is just like in CloD (at least on the Blitz version) you can "force" the game to accept joystick buttons as a modifier but you have to do it manually. Under "...steamapps\common\IL-2 Sturmovik Battle of Stalingrad\data\input" there should be a couple of files.
I modified 2:
Now I didn't experiment enough nor deep enough to be sure If I had to modify both of them but I did it and it worked. For example, I like to use the button 8 of my Hotas.X with my Thrustmaster 16000m´s Trigger as the wheel brake so you should look for the wheel brake command and it should look something like this:
action("rpc_wheel_brakes_p","State","key_b"); <---this line is the Key B on my Keyboard (just as an example)
action("rpc_wheel_brakes_p","State","joy1_b7+joy0_b0"); <---this one is the HOTAS.X , which on my case its joy1 and _b7 is the button 8 (the numbers on the HOTAS.X start with 1 but on the game they start with 0) + joy0 (my T.16000m) _b0 (the Trigger)
rpc_wheel_brakes, key_b, 0| // Wheel brakes <----again this line is the "B" Key on the keyboard
rpc_wheel_brakes, joy1_b7+joy0_b0, 0| <---this one is the HOTAS.X + T.16000M
And that's it, just save the changes and be sure you made some backup copies! also, keep in mind the joystick´s number can be different from mine. I'm sorry if this information was already posted somewhere else, but as I said this thread was the first it came up so I don't think it hurts to have it here as well. I really hope the devs make this whole process easier. Here is an in-game capture: