Take a look at this article in which Kunal
Chowdhury demonstrates a step-by-step tutorial on opening a Microsoft Excel book followed by data sharing between the Silverlight application and the Excel Sheet.
Here we will use a DataGrid which will load some customer information. Then we will pass the data to the Excel Sheet and then we will modify the data in the external application (i.e. inside the Excel sheet). You will see that, the modified data will reflect automatically to the Silverlight Application.