The current version of Photos (that came with with El Capitan) has the ability to use Extensions. Not quite the same, but it does allow you to manipulate your images with tools from external editors within the Photos Window. However this requires that a: the editor is purchased through the Mac App Store, and b: that the developer has written a hook for their app to make it available within Photos.
AT time of writing there are four such apps on the App Store and, in my testing, it works well. But it's not the same as the external editor feature in iPhoto.
Your options: work within the above limitations or use another photo manager.
The Photos for Mac forum is here
Photos for Mac