Relative Coordinates for Steering a Swimming Creature

We have simulated the motion of a multicelled swimmer, siphonophore, using kangaroo 0.99 that is driven by an MD Slider, so its cells get force vectors that propel the whole body.  Currently it will move in absolute xy coordinates: North, East, South, and West directions.  We would prefer to drive it like a joystick with relative coordinates to the body of the creature, but when we try to reference the position of the body in motion we create an infinite loop and does not work.