How to add config transform for custom config file?

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Visual Studio natively supports adding custom config transforms for web.config. Is there a way to do the same thing for a custom config file? For instance, the shop I'm at has an \[code\]EnvironmentConfigs\[/code\] directory in the root of the project and there are custom \[code\]appSettings.config\[/code\] and \[code\]connectionStrings.config\[/code\] files. Is there a way to do the same thing that web.config does and add \[code\]appSettings.Release.config\[/code\] and \[code\]connectionStrings.Release.config\[/code\] transformation files that will transform them on publish?