These instructions are for Visual Studio 2008. Might work for other versions as well.

- Open the website project in Visual Studio
- Click on File Menu
- Click on Add
- Click on Existing Project. Browse to your Silverlight project
- Click Open
- Right click on your Website Project on Solution Explorer
- Click on Property Pages
- Click on Silverlight Applications
- For “Use an Existing Silverlight project in the solution” you should see the silverlight project you added earlier on the “project” dropdown menu. Select the Silverlight project. Check “Add a testpage that references the control” if you like.
- Click on Add

From Webcosmo Webmaster Forum http://www.webcosmoforums.com/asp/21221-how-add-existing-silverlight-project-existing-website-project.html