If you want to work on a file or image, you can import it into Procreate from the Actions menu > Add > Image/File.
Also, remember that the iPad offers you the possibility to view two windows at the same time, so if you want to import an image to Procreate from Google, for example, just open it in the other window and hold it down to copy and drag it to Procreate.
You can also import PSD files to Procreate without any problem. This is one of the great advantages for designers who usually work in Photoshop, but do not have a computer at hand. With Procreate you can advance in your project without problems.