User.add_to_class('gender', models.CharField(max_length=1, choices=GENDER_CHOICES))
# и для отображения и возможности редактирования полей в админке:
User._meta.admin.fields += (
    ('Additional', {'fields': ('gender',)}),
)
User._meta.admin.list_display += ('gender', )

 File "/home/sv/django/wap/../../models.py", line 155, in <module>
    User._meta.admin.fields += (
AttributeError: 'NoneType' object has no attribute 'fields'

>>> from django.db import models
>>> from django.contrib.auth.models import User
>>> dir(User._meta.admin)
['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__']
>>>

class UserInline(admin.StackedInline):
    model = User

class ProfileAdmin(admin.ModelAdmin):
    inlines = [
        UserInline,
    ]

admin.site.register(Profile, ProfileAdmin)

Request Method:  	GET
Request URL: 	http://localhost:8000/admin/profile/profile/add/
Exception Type: 	Exception
Exception Value: 	

<class 'django.contrib.auth.models.User'> has no ForeignKey to <class 'profile.models.Profile'>

Exception Location: 	C:\Program Files\Python25\lib\site-packages\django\forms\models.py in _get_foreign_key, line 806