Symfony 2 merge security definitions from a bundle to the main app

I am creating a Admin Panel bundle and it uses some security settings - setting up firewall, ACL, authentication providers etc. At the moment the firewall and ACL are in the main security.xml in /app/config folder because when I put them in my bundle security.xml I get errors. What I would like is to decouple the bundle from the application so it is "pluggable" in other Symfony apps without the importing of the security configuration to the main security.xml every time. I read about the Configuration Tree builder and tried to put the values there but when I remove them from the security.xml it throws an error as well. Am I missing something here - do I need to do an additional step after building the configuration. I read the documentation but I could not find how to merge my security.xml in the bundle with the main app security.xml.

The exact error when splitting the configurat is:

InvalidConfigurationException: You are not allowed to define new elements for path "security.firewalls". Please define all elements for this path in one config file.

Any help would be appreciated.