I can't really say if it the return of __members__ is an implementation detail as there's no mention of that in the doc, but from reading the doc I think it's reasonable to think this is allowed:

import enum

class Colors(enum.Enum):
    RED = enum.auto()
    BLUE = enum.auto()

Colors.__members__['GREEN'] = enum.auto()

And the traceback:

Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    Colors.__members__['GREEN'] = enum.auto()
TypeError: 'mappingproxy' object does not support item assignment

is somewhat confusing, as the documentation says an "ordered dictionary" is to be returned, and the traceback talks about a mapping proxy.