[dannythefool]

I don't think it's possible to do that directly, but what you can do is make a new bezier object with the corners rounded as you want them, and then you can apply this shape to any other existing rectangles in the same document (different canvas works). They show up with blue borders in the list with the other pre-defined shapes.

Drawing these bezier objects is easiest with snap to grid enabled, and the grid visible. Just click on the corners in order to create straight lines. For corners that should be rounded, click a few grid points short of the corner, then drag to the actual corner. Then do the same on the adjacent side of the rectangle so it's symmetrical (that way the corner will scale properly). You can always adjust the curve later but it'll only affect the shape you're editing, so if you have to adjust it you'll have to re-apply it to all affected boxes.