The Compact .NET Framework doesn't support the same amount of functionality as the full Framework (for size and platform reasons) so you may find that some, but not necessarily all, of your code may paste into a Smart Device project. Not all controls available to a Windows Forms app are available to a Smart Device app either.
I don't think it is possible to simply import a Windows app into a Smart Device app.
You will almost certainly have to make a number of changes anyway to allow for the different screen size and input methods.