You can save the custom shapes (png files) in any directory on your PC; they will keep their « masks » properties. You could then access them through the Browse button.
However if you want to access them in the same way than with the Scrapbook MAX! shapes, you could save them at the same location.
C:\Users\UserName\Documents\Scrapbook MAX! 2.0\Gallery\Images\Shapes
If you create a folder called « MyShapes » :
C:\Users\UserName\Documents\Scrapbook MAX! 2.0\Gallery\Images\Shapes\MyShapes\, then when you add a photo and/or a paperscrap and you select « Custom Shapes », you will see the category « MyShapes » in the Properties dialog.