I have never been able to successfully add a Form from an existing file to a new project.
Form
I read on a blog that you add just the *.cs file and the depen
You can still use "Add Existing iTem"
"Add Existing iTem", select three files (.cs .resx .Designer.cs)
make sure "namespace Application" in your .cs file is the same as the project
now you can use auto-complete the Form you just added.