request.POST = request.POST.copy()
;qd = QueryDict('', 'mutable')
qd[key] = value
request.POST = qd
request.POST['new_key'] = 'some value'
my_form = MyModelForm(request.POST)
my_form.is_valid()
my_model = my_form.save(commit=False)
my_model.pass = 'foo'
my_model.save()