Might be an interesting dilema there, bromensch. Though that sounds like a really odd choice for a control scheme, I'm not going to pick at that.
The inherent problem is that the mouse's x- and y-axes, as far as I know, are the same as a stick's axis. I'm assuming you want to use your stick to yaw and pitch your fighter, while you want to use the mouse to roll it. Again, the issue is you have shared axes between your mouse and your stick. That said, there might be an irregular solution around this, but I'm not sure a standard solution is in order.
Now, if that's not the case, and you've an even more peculiar control scheme in mind, you should just be able to select the bank axis control binding and move the mouse in the x-axis to bind roll to that axis. I could be wrong, though. It's been a while since I've needed to bind flight controls.