The thing that is hard for this kind of behavior is to detect the conveyor belt. 'Cause when the cube is on it, it doesn't overlap it so you can't really know the nature of the 'ground' your on.
So I used another Sprite for detection. I pined it to the box and made it slightly bigger so it sticks out of the cube (you can see that if you set the boxColl object to visible in properties)
When the box detects the conveyor belt, it takes its velocity instance variable and put it in the move instance variable of the box.
And as soon as the move instance variable isn't equal to 0, bam it moves.
Oh... Well I don't need the 'move' variable I can directly plug the velocity to the box, you can re download the .capx to see the difference if you want.
The neat things is that you can have conveyor belts with different speeds