Installing Passenger when Nginx is already installed; Possible?

In Passenger docs the chapter "Generic installation, upgrade and downgrade method: via RubyGems" discusses this. Basically, once the Passenger gem is installed, nginx needs to be recompiled (and then used instead of the yum/apt-get-installed nginx if one exists). Passenger's compilation/configuration utility "passenger-install-nginx-module" does it for you (it's part of the Passenger gem), and it automatically includes the necessary switches for Passenger. It also gives you the option to add your own switches (such as for extra modules, or to enable/disable NGiNX's built-in features).

I think your problem is that the passenger module is not present in nginx.

All the passenger dependent directives you've described (passenger_root, passenger_ruby, passenger_enabled) are available only when the passenger module is attached to nginx. This is why you have to compile nginx with --add-module='/path/to/passenger-3.0.9/ext/nginx'.

Unfortunately, I don't know of any method to enable passenger module without re-installing nginx. But, according to http://wiki.nginx.org/Modules, "Nginx modules must be selected at compile-time.", so there could be a chance that there isn't a way to do that.