According to the "alt" field shown in the IBM link provided by @luistm , I was able to reproduce something like this in Dia:
The "alt" field was drawn using the UML Large Package object. (A small package does not allow resizing.)
Condition labels are only Text objects (found on the top panel [static] on the left toolbar, marked with a T).
A dashed line is only a Line object (from the same panel of static toolbars), while the arrow is set to a regular line, and the body is set to a dotted line (made at the bottom of the left toolbar with these 3 arrows).
I had to play with the menu items “Objects”, “Send Back”, “Bring to the Front”, “Send Back” and “Bring Forward”, so that everything was correct, without obstacles to each other. Also, I often had to use the Refresh Menu command to check myself; often the drawing will ruin the stacking order and overlap, but Refresh will return things again.
This is not like the diagrams on the IBM link (the label is outside the main window, not inside, the frame is marked with a life line, etc.), but I think this is the best we can do with UML Dia objects.