It’s definitely possible to access the images in the Photos library in that manner. For example it’s possible to open the photo library package in the Finder and drag the “originals” directory to RAW Power, which would just show you the originals instead of all the thumbnails and whatnot. However, that’s not the intent of the “Photo Library” feature. The Photo Library feature gives you access to synchronized ratings, non-destructive edits that sync over iCloud, integration with the RAW Power Photos extension, and the ability to manipulate the photos library as well.
I should be clear that working with the Photo library manner by accessing internal directories is an unsupported configuration, because Apple does not support people messing around inside the Photo library. You have to be extremely careful if you choose to use the app in this manner. Definitely do not delete any originals, for example (or use Move to Trash). I do not know what else might not work.