Sometimes it is useful to leverage a general Visual Studio Project up to a Windows Workflow Project. To setup Workflow-Designer-support, a manual modification of the targeted ".csproj"-file is needed. Otherwise errors come up like this:

"The service 'System.Workflow.ComponentModel.Design.IIdentifierCreationService' must be installed for this operation to succeed. Ensure that this service is available. "

The entry for the project-guid has to be replaced by this:

<ProjectTypeGuids>{14822709-B5A1-4724-98CA-57A101D1B079};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>