Basically, check in lib/pf/web.pm, there are two subs:

generate_registration_page
generate_login_page

and these are called from cgi-bin/register.cgi.

Now, your goal is to skip generate_registration_page.

Add the following line to html/user/content/templates/login.html anywhere between <form..>...</form> 
tags:
<input type="hidden" name="mode" value="register">

and patch cgi-bin/register.cgi with:
--- pf/cgi-bin/register.cgi    9ad54d294120b0f8e3e63c6e6f36109679e9f41d
+++ pf/cgi-bin/register.cgi    18c03de41b3c6625fc52baf268de167585375c02
@@ -145,5 +145,5 @@ if (defined($params{'mode'})) {
     generate_error_page($cgi, $session, "error: incorrect mode");
   }
 } else {
-  generate_registration_page($cgi, $session, $destination_url, $mac,1);
+  generate_login_page($cgi, $session, $ENV{REQUEST_URI}, $destination_url, $err);
 }

This should do the trick but note that I have not tested it and something might be missing.