<extension point="org.eclipse.help.contentExtension">
  <contentExtension
     file="extension.xml">
  </contentExtension>
</extension>

<?xml version="1.0" encoding="UTF-8"?>
<contentExtension>
  <replacement
    content="replacement.xhtml#newElement"
    path="/pluginA/myTopic.xhtml#elementId">
  </replacement>
</contentExtension>

<extension point="org.eclipse.help.contentExtension">
  <contentExtensionProvider
    class="pluginB.provider.MyContentExtensionProvider">
  </contentExtensionProvider>
</extension>

public class MyContentExtensionProvider extends AbstractContentExtensionProvider {
  @Override
  public IContentExtension[] getContentExtensions(String locale) {
    return new IContentExtension[] { createExtension() };
  }

  private IContentExtension createExtension() {
    return new IContentExtension() {
      @Override
      public boolean isEnabled(IEvaluationContext context) {
        return true;
      }

      @Override
      public IUAElement[] getChildren() {
        return new IUAElement[0];
      }

      @Override
      public String getContent() {
        return "replacement.xhtml#newElement";
      }

      @Override
      public String getPath() {
        return "/pluginA/myTopic.xhtml#elementId";
      }

      @Override
      public int getType() {
        return REPLACEMENT;
      }
    };
  }
}