Not, "out of the box", but it's pretty easy. Basically, just add:

my $s = $in->param('s') | | $in->cookie('s');
my $user = &authenticate($s);
if (!$user) {
... # error page/redirect to login
}
else {
... # rest of add or modify.cgi
}

You just want to make sure a user is logged in before you allow them to do anything in add or modify.

Hope that helps,

Alex