It actually goes into plenty of detail about how to add drop areas into your own Uniface forms, which is something I'd definitely like to try some time. But what really interested me was the opening paragraph...
What very little people know is that in the Import Repository Objects component there is a small section where you can drag your export files from Windows Explorer. After you drop them, the File name box is updated with the list of files that you dropped. The red section is where the drag-and-drop field is located.
I tried this today and it definitely works. You learn something new every day!
Summary: You can drag and drop export files onto the Import Repository Objects popup, which is really quick and convenient.