[Plugin: WP Photo Album Plus] adding watermarks to photos already uploaded

Is there a way to add a watermark to photos that were already uploaded before the watermark function was added to the plug-in?

No, not directly.

The easyest way is to update them (upload with a ftp program to your depot dir and import with the update checkmark on).
The filenames should be equal to the existing photo names, and there should be only one copu of the photos in the wppa+ system.

Thank you ??