2.) Get strong name with JustDecompile e.g.:
Telerik.Web.UI, Version=2012.2.607.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4
3.) Replace "Assembly" on UserControl with strong name:
<%@ Register Assembly="Telerik.Web.UI, Version=2012.2.607.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4" ...
4.) Open Package.package -> Advanced -> Add -> Add existing assembly...
4.a) Source Path, e.g: C:\Program Files (x86)\Telerik\RadControls for ASP.NET AJAX Q2 2012\Bin35\Telerik.Web.UI.dll
4.b) Deployment Target: GlobalAssemblyCache
4.c) Location: Telerik.Web.UI.dll
4.d) Safe Controls: -> Click here to add new item...
4.e) Namespace: Telerik.Web.UI
4.f) TypeName: *
4.g) Assembly Name (e.g.): Telerik.Web.UI, Version=2012.2.607.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4
4.f) Safe: =Checked