I believe you can use the metadata templates for this purpose, but the digiKam documentation is not exactly clear on this.

According to section 2.4.3 of the digiKam Handbook:

At the very least, for all images that will be exported and/or published in any form, the authorship and copyright information should be part of their metadata. Nothing more simple to do with digiKam: you can setup the default identity, and any images ingested be digiKam will be automatically informed.

I believe this is referring to the metadata templates which you can define by opening digiKam then selecting the "Settings" menu, then "Configure digiKam". In the window that appears, select the "Templates" icon from the scrolling menu on the left. You should now see the "Metadata templates" page of digiKam's settings, and you can define copyright and author information here. There are also tabs for "Location", "Contact" and "Subjects" but I don't know why you would want these values to be appended to all imported images.

I recommend you run some tests to check that you get the result you want with a few test images, before going ahead with a mass import of your image library.