from webtest.forms import Field
# ...
extra_field = Field(form, tag='input', name='new_field', pos=0, value='new_value')
form.fields['new_field'] = [extra_field]
form['new_field'] = 'new_value'
form.field_order.append(('new_field', extra_field))
# How to add a form to formset: https://github.com/kmike/django-webtest/pull/17