Interruptible Activity Region is not available on the tool bar. So to represent it in Astah, please use the free shape - rounded rectangle.
(1) Choose [Rounded Rectangle] from tool bar.
(2) The shape appears on the diagram. By default, it appears as a solid line. Right-click the rectangle and go to [Line Type] and select dashed line.
(3) Locate it to cover the region
Hope this helps.