The following recipe is valid for any type of settings supported by Symfony framework.

imports:
    # Let's import our template selection rules that reside in our custom bundle.
    # MyCustomBundle is the actual bundle name
    - {resource: "@AcmeTestBundle/Resources/config/templates_rules.yml"}
 
ezpublish:
    # ...

# Here I need to reproduce the right configuration tree.
# It will be merged with the main one
ezpublish:
    system:
        my_siteaccess:
            ezpage:
                layouts:
                    2ZonesLayout1:
                        name: "2 zones (layout 1)"
                        template: "AcmeTestBundle:zone:2zoneslayout1.html.twig"
    
            content_view:
                full:
                    article_test:
                        template: "AcmeTestBundle:full:article_test.html.twig"
                        match:
                            Id\Location: [144,149]
                    another_test:
                        template: "::another_test.html.twig"
                        match:
                            Id\Content: 142
    
            block_view:
                campaign:
                    template: "AcmeTestBundle:block:campaign.html.twig"
                    match:
                        Type: "Campaign"

During the merge process, if the imported configuration files contain entries that are already defined in the main configuration file, they will override them.

If you want to import configuration for development use only, you can do so in your ezpublish_dev.yml 

<?php
 
namespace Acme\TestBundle\DependencyInjection;
 
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\Config\Resource\FileResource;
use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\Extension\PrependExtensionInterface;
use Symfony\Component\HttpKernel\DependencyInjection\Extension;
use Symfony\Component\DependencyInjection\Loader;
use Symfony\Component\Yaml\Yaml;
 
/**
 * This is the class that loads and manages your bundle configuration
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/extension.html}
 */
class AcmeTestExtension extends Extension implements PrependExtensionInterface
{
    // ...
 
    /**
     * Allow an extension to prepend the extension configurations.
     * Here we will load our template selection rules.

     *
     * @param ContainerBuilder $container
     */
    public function prepend( ContainerBuilder $container )
    {
        // Loading our YAML file containing our template rules
        $configFile = __DIR__ . '/../Resources/config/template_rules.yml';
        $config = Yaml::parse( file_get_contents( $configFile ) );
        // We explicitly prepend loaded configuration for "ezpublish" namespace.
        // So it will be placed under the "ezpublish" configuration key, like in ezpublish.yml.
        $container->prependExtensionConfig( 'ezpublish', $config );
        $container->addResource( new FileResource( $configFile ) );
    }
}

# We explicitly prepend config for "ezpublish" namespace in service container extension, 
# so no need to repeat it here
system:
    ezdemo_frontend_group:
        ezpage:
            layouts:
                2ZonesLayout1:
                    name: "2 zones (layout 1)"
                    template: "AcmeTestBundle:zone:2zoneslayout1.html.twig"
 
        content_view:
            full:
                article_test:
                    template: "AcmeTestBundle:full:article_test.html.twig"
                    match:
                        Id\Location: 144
                another_test:
                    template: "::another_test.html.twig"
                    match:
                        Id\Content: 142
 
        block_view:
            campaign:
                template: "AcmeTestBundle:block:campaign.html.twig"
                match:
                    Type: "Campaign"

Service container extensions are called only when the container is being compiled, so there is nothing to worry about regarding performance.

Configuration loaded this way will be overridden by the main ezplatform.yml file.