title: How to override PropertySourcesPropertyResolver and add an additional property source to Externalized configuration
body: <p>Is there a way to override Spring Boot's PropertySourcesPropertyResolver and extend <a href="http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#boot-features-external-config" rel="nofollow">Externalize configuration</a> by adding additional property source?</p>
  
  <p>What I'm trying to do is to add another property source in the current list, and be able to override this property with current mechanism.
  And to extend PropertySourcesPropertyResolver, so when Spring is mapping properties for classes annotated with @ConfigurationProperties
  and requesting for the key, it can check for key with two different prefixes.</p>
  
  <p>For example, if I have properties from two different locations:</p>
  
  <p>Properties on location 1:
    data.user=userName</p>
  
  <p>Properties on location 2:
   service.data.user=serviceUserName</p>
  
  <p>I want to be able to override the value of data.user with a value of service.data.user (if that property exists).</p>