You can send images from the Photos app to edit in Pixelmator Pro and then save the changes back to the original image in your library.
Do one of the following:
To save your changes back to the original image, choose File > Save in Pixelmator Pro.
When saving images back to Photos, Pixelmator Pro does it in the JPEG file format by default, so all the layers, adjustments, effects, styles, and other formatting you add to the image are merged. To keep the nondestructive edits and layers, choose File > Export (from the File menu at the top of your screen) and export the image to the Pixelmator Pro file format. You can also edit your photos right in the Photos app using the Pixelmator Pro Photos extension.