If you have a excel file within your application , you can launch the file and open it in Microsoft excel from a Windows Phone App using the Launcher.LaunchFileAsync method.

How to Launch Excel File from a Windows Phone 8 App ?

Below is a code snippet demonstrating how to launch the excel file from a Windows Phone 8 App.

var excelfile = await Windows.ApplicationModel.Package.Current.InstalledLocation.GetFileAsync("Ginktage.xls");
Windows.System.Launcher.LaunchFileAsync(excelfile);