To answer to comments asking to move only some settings to a custom file
I found the file
attribute that should help:
in web.config:
...
<appSettings file="otherSettings.config">
<add key="owin:AutomaticAppStartup" value="false" />
<add key="Environment" value="Test" />
</appSettings>
in otherSettings.config:
<appSettings>
<add key="Usuer" value="myUser" />
<add key="Password" value="thePassword" />
</appSettings>
I tried and the app reads both settings in the web.config file and in the otherSettings.config. HTH