Is it possible to add a roll function to the UFO camera, in addition to the pitch, yaw, tracking and dolly controls? I'm using an XBox 360 controller to operate the free-cam. The right stick controls pitch and yaw, and the left stick controls horizontal tracking and dolly movement, but there's no roll. Both the left and right triggers and bumpers control vertical tracking, but one of those could be set to control camera's left-right roll, instead of both doing the same thing; and pressing both right and left simultaneously would reset the camera roll back to level.

Is something like that possible, or not?

There is indeed no roll. The only way to roll the camera is with an animation.