User can import existing eclipse4SL resource following below.
Right click the Project and select Import.
In the Import dialog, select Silverlight->Existing File.
Browsing the computer and find the resource directory. Checked the resource which you need to add.
Note: In general, when a XAML (*.xaml) is selected, the partial class (*.xaml.cs) will be imported automatically if there is such file under the same folder.