Slicer application’s Python environment is your “user-configured Python environment”. You can configure it as any other Python environments. As @muratmaga described, it would be impractical for you to set up and maintain a custom Python environment that is compatible with the Slicer application.
However, if you just want to import Slicer as a library into your Python environment and use some Slicer features from there (some widgets, algorithms, etc.) then you will be able to use SlicerLib. It is planned to be released around the end of this year.