[Answered ]-Can I extend a function based view?

You can use variable-length arguments:

def my_activate(request, *args, **kwargs):
    res = activate(request, *args, **kwargs)
    # Do stuff you need to do
    return res

Specifically to your problem, I think, in your URL definition, you should be passing success_url in a dictionary:

url(r'^activate/(?P<activation_key>\w+)/$',
    userena_views.activate,
    {'success_url': '/foo/'}, name='userena_activate',),

(Just a guess, that’s usually how additional parameters are passed to view functions.)