You cannot add your own shapes or clipart to the supplied pallets at this time.  

You can import images into 'BoilerpPlate' for inclusion  the Process Document export and into the Activity Diagram canvas area.   These are not re-usable in that they must be re-imported each time and you cannot add additional data to the image imported, although you can position it where you want on the canvas and re-size it to fit appropriately in the flow diagram.