Hey dielser, yes bodies are centered, usually on the center of mass, that's how things conventionally are for physics.
So instead of changing that, you should work around it but its simple :
If I understand correctly your first touch should define the top left corner of the platform and the final touch (end phase of a touch) should define the bottom right then the platform needs to be created?
the position of your platform needs to be the center of firstTouch and finalTouch points in local coordinates and width is finalTouch.x - firstTouch.x and height is finalTouch.y - firstTouch.y .
edit :with physics engines anyway, you can change the origins of bodies but that means changing the center of mass I believe which is not recommended for a good simulation - but I guess for a static element like a platform it wouldn't matter much, but I don't know how to do that, citrus engine combines other frameworks so even though to reply to questions at my best I need overall knowledge (and to contribute as well) anyone specifically using box2D or nape should read their docs to get better info that are not only guesses.