dropbox.com/s/3mbjkjh35w4g98u/physicspushingexample.c3p
Use an invisible helper sprite to define a "pushing" zone. Although this method is more suitable for platform behavior. This example uses 0 friction surfaces, and defining the physics velocity on a fixed, immobile surface. This will probably not be ideal in a situation where other outside forces will act upon the objects in question.
Another way to get constant motion is to apply a force exactly equal to the amount of friction acting upon the object. I don't know how to get that number.
To do it properly in a full physics environment, your conveyor should actually consist of moving elements, and apply forces on objects resting on them via friction.