Sounds like you are trying to create a Class Library for reference within your project. Typically app.config files are applied by default to an executed assembly (like a console application or a forms application, or a web application in the case of a web.config file). These app.config files are really just plain xml configuration files that .NET framework classes happen to provide facilitated access to (via classes like ConfigurationManager). The only thing that is somewhat special about app.config is that the file is renamed to have the same name as its assembly after compilation (ie if your project is called MyProject.dll, the config will be called MyProject.dll.config). All that said, none of this is necessary to configure your project using a configuration file.

For your own assemblies, you can just as well use a different config file that behaves just like the app.config, and then map to that config file. The method you want to use is ConfigurationManager.OpenMappedExeConfiguration (though there are many others you can also use to get the job done).  Here is a link to the MSDN, though you might want to look for other references too https://msdn.microsoft.com/en-us/library/ms134269(v=vs.110).aspx

You could of course just not use ConfigurationManager, and instead configure this yourself in any arbitrary .xml or .txt file and use basic file I/O; as long as the files are somewhere the Workflow Server can access, there is nothing stopping you from reading/writing a file.