Further to above, you can extend the canvas. Open the picture file, then go to Image>Resize>Canvas size. In the dialog that comes up, extend the canvas height to 125%, for example. Make the canvas extension white. Click on the 12 o'clock anchor to have the extension below the picture. ok it. Now, activate the type tool and type the identifying information. This will avoid superimposing the text on the picture.
Mike, if you are asking about saving a caption and copyright information to metadata, the answer is that you can do so in the Organizer.
After selecting an image in the grid, the General tab of the Information panel has a text box for a caption. If an image in the Organizer is less than full screen view, you can right click to get a context menu to Add Caption. In full screen view, a text box becomes available when clicking below the image to add a caption.
As for adding copyright information to the metadata, this can also be done in the Organizer's Information panel (1). You have to click on the elipsis next to the IPTC button (2). This opens a new dialog (3) with a text box for Copyright information (4).
In the Photoshop Editor, you can add metadata to an open image by clicking on File>File Info . . . This will bring up another dialog where you can add information such as captions and copyright information.
Also in the Editor, it is possible to add captions and watermarks to multiple files by using the File>Process Multiple Files tool.