Moving a custom configuration group to a separate file

I've recently wrote a rather large custom configuration group. I'm curious if it is possible to move this configuration to a separate file via the following:

<configuration>
    <configSections>
        <sectionGroup name="MyCustomGroup">
            <section name="MyCustomSection"/>
        </sectionGroup>
    </configSections>
    <MyCustomGroup file="alt.config" />
</configuration>

This is something similar to what you can do with the file attribute for appSettings. I realize there is most likely a need to create a ConfigurationPropertyAttribute for my custom section handler, however I've been unsuccessful in finding any example or direction in this regard.