Automatically converting any Flash movie to Silverlight is a complicated task since Flash player and Silverlight runtime have different programming API and programming languages. But there are two tools that could help you with this task - SilverX and SilverBoom.
However, Flash movies without ActionScript and interactivity can be automatically transformed to Silverlight using two similar tools: SilverX - desktop application and SilverBoom - online service.
These tools take SWF file as a input and convert it to Silverlight solution or Silverlight XAP file (ready to run). Of course, these tools can not completely solve Flash to Silverlight migration task, but provide good Silverlight output on animations/cartoons and banners.