http://chriskief.com/2012/12/17/subclass-a-django-model-form-and-add-extra-fields/

forms.py

from django import forms
from base import *
from app.models import User


class RegisterEmailForm(RegisterBaseForm):

    first_name = forms.CharField(max_length=User._meta.get_field('first_name').max_length)
    last_name = forms.CharField(max_length=User._meta.get_field('last_name').max_length)

    class Meta(RegisterBaseForm.Meta):
        fields = RegisterBaseForm.Meta.fields + ('first_name', 'last_name')