Adding new fields in ModelAdmin with fieldsets to edit user, and add_fieldsets whan creating a new user
from django.contrib.auth.admin import UserAdmin
class CustomUserAdmin(UserAdmin):
...
fieldsets = UserAdmin.fieldsets + (
(None, {'fields': ('custom_field',)}),
)
add_fieldsets = UserAdmin.add_fieldsets + (
(None, {'fields': ('custom_field',)}),
)