They would not negate each other. The force generated by the plane is via the air (air forced through the turbines)....the force generated by the conveyor belt is transferred to the wheels, which in the instance of take-off force play no part.

The air being forced through the turbines will be pushed through those turbines regardless of the conveyor belt's speed. And therefore, the plane will respond, as it does in take-off scenarios, to that force through the turbines. And it will react by moving forward through the airspace it is in, and the airspace that is being affected by the turbines' forcing of said air. The wheels' operations and reactions to the conveyor beneath them are unimportant in this hypothetical scenario.

The only way the conveyor could affect the plane's ability to lift off under take-off thrust was if the conveyor could also negatively (with relation to the plane's intended forward motion) affect the air through which the plane needs to move to achieve lift-off velocity.