Hmmm... Maybe you could try using a LinearMotor with a solid on the end. The solid could be retraced inside the robot normally and extended out when necessary. That would probably be the closest analog to a real-world robot.

If you'd rather sacrifice realism (i.e. this barrier is part of a larger simulation and not meant to be a robot in its own right), you could also setup a Solid with a Box geometry and use the supervisor API to edit the box's size. Although that's a little hacky.