for me it looks like that there is currently no way to extend an existing configuration file by installing a feature. But this is, what I would need for my use case and what would be great IMHO for other users as well.
Here is my idea.
If a property already exists within a configuration file this property should not be changed. But if a configuration file does not contain a certain key, this key=value should be added at the end of the configuration file.
If com.foo.bar.cfg does not contain myProperty before the feature containing this example was installed, then myProperty=myValue should be added to com.foo.bar.cfg file.
This would make it possible to extend existing configuration files for custom needs. A general use-case could be logging. My application would like to add a logger configuration in case this logger is not configured already.
<config name="org.ops4j.pax.logging"> log4j.logger.org.apache.activemq.broker.util.LoggingBrokerPlugin=INFO, stdout, osgi:VmLogAppender