To import new files into an already existing project in Spine without creating a new project, you should ensure that the "New project" option is unchecked during the import process. This allows you to import data directly into the current project. If you're importing images as attachments, make sure to place them in the skeleton's images directory and then use the appropriate option to create new attachments for these images within your project. For importing other types of data, like skeletons or animations from another Spine project, use the "Import Project" feature from the main menu and select the specific elements you want to import into your existing project.

You can add image files and the create region attachments by dragging them into the viewport or to a bone or slot.

Or you can use import data or import PSD to add to your existing skeleton, just make sure to uncheck New project.