Yes, it's possible to split webapp's own /WEB-INF/faces-config.xml file's contents over multiple files. You only need to alter the web.xml to specify the javax.faces.CONFIG_FILES context parameter with comma separated webcontent-relative paths to all external files:
<context-param>
    <param-name>javax.faces.application.CONFIG_FILES</param-name>
    <param-value>/WEB-INF/navigation-config.xml</param-value>
</context-param>
Alternatively, if you're already on JSF 2.x, then you could also get rid of all navigation cases altogether by utilizing the JSF 2.0 new implicit navigation feature. I.e. just make the from-outcome exactly the same as to-view-id as in return "/result.xhtml"; or return "/result"; instead of return "success";. This way the whole navigation case becomes superflous and can be removed from faces-config.xml.