Using ConfigurationManager to write to an external config file?

Question

I've not been able to find any easy reference on this:

I have a web.config file that specifies <appSettings file="someotherfile.config"/> to put settings in an external file. That means that someotherfile.config is not a "valid" configuration file, as it doesn't follow the full tree format, it just contains the appSettings subtree.

Is there any way to use the normal ConfigurationManager APIs to write settings into this file? (I realize I can just treat it as an XML file and manipulate it that way)

Thanks