When I was working in a normal c# class project I wanted to create a new workflow class item. Unfortunately the template wasn't available.

Luckily I found the post from Eric Schoonover which tells you basically four steps:

1. open your csproj project file in notepad
2. copy the following tag in the <AssemblyName> section:
   <ProjectTypeGuids>{14822709-B5A1-4724-98CA-57A101D1B079};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
3. Copy the following import node as the other existing import nodes:
   <Import Project="$(MSBuildExtensionsPath)\Microsoft\Windows Workflow Foundation\v3.0\Workflow.Targets" />
4. save the file and reload your solution
5. Make sure you include the correct reference in your project:
   e.g microsoft.office.workflow.tasks