Oh ffs
entityIn.setVelocity(v.x - 2.0D, v.y, v.z);
Though you'll probably have to fiddle with the 2 value (probably something more like 0.2). Because think about what should happen if the player stands still (the conveyor belt moves him at a constant speed) and what happens if the player moves against the belt (the player should move slowly, which direction is up to you, but either way the two velocities are fighting) and what happens if the player moves in the same direction as the belt (the player should move faster).
Right now you're only getting "the player moves faster." But that faster is either "in the direction the player is already moving (your original code) or "in the direction the belt is moving" (what poopdice suggested) so a player trying to go against the flow of the belt gets rocketed backwards!