(Using AS3)

I'm doing a simple grid layout of boxes (movieClips), i'm then trying to save their current position to each of the movieClips.

So in AS2 i'd do something like ...

obj.origX = obj._x;
obj.origY = obj._y;

but it's saying;

1119: Access of possibly undefined property targetX through a reference with static type flash.displayisplayObject.

How can i add custom variables to movieClips?